SINO-AMERICAN relations have come of age. Twelve years after Richard Nixon's first visit to Peking, and four years after Deng Xiaoping's triumphal tour of the United States, the Chinese-American relationship is characterized neither by hostility nor by euphoria.
Instead, both sides have a sober appreciation of the fundamental importance of this relationship in a turbulent and uncertain world. They know the things that unite them as well as the nature and limits of their continuing disagreements.
''Promotion of Sino-United States friendship is the common desire of the two peoples and thus has deep roots,'' State Councilor Ji Pengfei wrote recently in the fortnightly magazine World Affairs.
''It is also in the interest of peace and stability in Asia and the rest of the world.''
Mr. Ji, a former vice-premier and foreign minister, is the Communist Party's top official in the foreign affairs field.
His sentiments will probably be echoed by Chairman Deng, General Secretary Hu Yaobang, and Premier Zhao Ziyang when Ronald Reagan sits down one by one with the Chinese leadership during his April visit to Peking.
The Chinese leaders will be frank about the ''difficulties and obstacles'' Mr. Ji cites. The principal obstacle is, of course, Taiwan, with which Peking seeks reunification while Washington continues to give Taiwan the means to defend itself militarily.
The Chinese leaders are well aware of President Reagan's feelings toward Taiwan. He has acknowledged the various communiques signed by his predecessors, whereby the US recognizes but one China, the People's Republic, and also recognizes that Taiwan is a part of China. But the President continues to express friendship for ''the people in Taiwan'' and says he is not about to abandon an old friend (Taiwan) to make a new one (Peking).
At the same time, the framework for containing the Chinese-American disagreement over Taiwan has been carefully worked out and is embodied not only in the Shanghai communique signed by President Nixon in 1972 and the agreement on diplomatic relations negotiated by President Carter at the end of 1978 but also in the communique of Aug. 17, 1982, on American arms sales to Taiwan.
This last agreement, reached after protracted, difficult negotiations, is open-ended but provides for the gradual diminution of such arms sales. The Chinese appear to have accepted that they cannot push Washington any further than they have in the Aug. 17 communique. They have shifted their emphasis to developing Chinese-American relations in other areas, notably the economic.
This was the main thrust of Premier Zhao's very successful visit to the US in January. He did not sidestep the Taiwan problem but never highlighted it.
His purpose, as a Chinese source expressed it, was not to change American views on Taiwan. Rather it was to gain American understanding for what China is trying to achieve in the way of modernization and economic growth, to reaffirm China's open-door policy toward cooperation with Western countries, and to convey a sense of confidence in China's political stability and in its ability to reach the goals it has set.
From this viewpoint, Peking is well satisfied with Mr. Zhao's visit. The purpose was not to reach new under-standings and agreements but, in the words of one Western diplomat, to ''fill in the framework already established.''
President Reagan's visit in April will have much the same purpose. Of course, it is bound to be a news media event. From the standpoint of domestic American politics, it will spotlight one of the few genuine foreign policy successes of the Reagan administration and will enable Republican candidate Ronald Reagan to look presidential while his Democratic opponents squabble with each other in their struggle to win the nomination of their party.
The Chinese know this. They know the importance of Mr. Reagan's visit at the start of his reelection campaign, and they will not try to score Brownie points off him.
''We know President Reagan cannot say anything very different about Taiwan from what he has been saying in the past,'' says a Chinese source well qualified in the foreign policy field.
Rather, the Chinese see the visit as symbolizing a kind of maturity in Chinese-American relations. This is no longer the season for Mr. Deng to be photographed wearing a cowboy hat in Texas, but rather for Mr. Zhao to be probing the secrets of California's Silicon Valley.
The Chinese leadership is reliably reported to be extremely interested in futurologist Alvin Toffler's work ''The Third Wave.'' They see that in the 1960s and early '70s, when newly industrializing countries like South Korea and Taiwan successfully caught and rode the wave of world economic growth, China, mired in the trauma of the Cultural Revolution, missed the wave.
They seem to be convinced that the Western world is about to experience a new industrial revolution, and this time they want to position their country so it can catch and ride this new wave. Otherwise, they fear, China will fall even further behind the industrialized countries of the West than it is today.
They note with envy that Taiwan, for instance, no longer relies on textiles as its principal export item, but rather on a whole array of sophisticated components for the electronics and computer industries. All this is a far cry from classical Marxism's claim that the capitalist world is bound to sink under the weight of its own contradictions.
CHINESE propaganda does hold high the banner of ''scientific socialism,'' and in political and social terms China remains a rigidly authoritarian, paternalistic, bureaucratic, Communist state. As regards the economy, the Chinese leadership comes close to recognizing that, within the framework of overall control by the Communist party, ''scientific socialism'' is whatever works.
True, industrial innovation and economic experimentation are often hobbled by the political and social constraints of the Confucian-Communist state and society that China is today. The leadership warns of the dangers of ultraleftism , meaning in economic terms, conservative Marxist thinking opposed to the innovation associated with people such as Premier Zhao or General Secretary Hu. It warns equally of ''rightism,'' thinking vulnerable to the ''sugarcoated bullets'' of bourgeois liberalism.
Every year that passes since the death of Mao Tse-tung and the overthrow of the ''gang of four'' headed by his widow Jiang Qing helps to solidify the rule of the present pragmatic leadership headed by Deng Xiaoping. More than seven years have gone by since Mao's passing and more than five years since the Deng line triumphed at the third plenum of the Central Committee in December 1978.
But the leadership must remain vigilant, knowing how often China's political line has changed in the past and conscious that the Chinese people are asking the same questions about the stability and permanence of the present line as do foreign businessmen and politicians.
A few months ago the leadership raised a hue and cry about mental pollution - a dilution of Communist values by borgeois liberal thinking imported from the West. Now it is once again emphasizing rectification - a rejuvenation and purification of party ranks by eliminating pockets of opposition to the Deng line.
Meanwhile, the Soviet Union remains a formidable foe and threat to China's security.
China's leaders emphasize they pursue an independent foreign policy and seek better relations with both the US and the Soviet Union. In his World Affairs article, Mr. Ji spoke of the ''traditional friendship between the Chinese and Soviet peoples'' and said it was ''China's consistent stand that the two countries should normalize their relations on the basis of the five principles of peaceful coexistence.''
This week China and the Soviet Union are holding the fourth round of talks aimed at improving their relations. Mr. Ji said he hoped for ''substantive progress'' in removing three obstacles to better relations often cited by Peking: Soviet troops in Afghanistan, Soviet support for Vietnam's occupation of Kampuchea, and the Soviet military buildup on the Sino-Soviet border and in Mongolia.
Chinese-Soviet relations have gradually improved since the first round of talks was held here in October 1982.
Two-way trade this year is expected to reach $1.2 billion. Moscow has offered to help modernize some Chinese factories originally built with Soviet equipment, and in May, Soviet Deputy Premier Ivan Arkhipov is expected here to discuss the offer in greater detail. Western diplomats here point out that it is in Washington's interest that Chinese-Soviet relations should be stabilized, that the tensions long prevailing along the frontier between China and the Soviet Union should be eased. Peking is not talking about renewing its alliance of the 1950s with Moscow, nor even about normalizing party-to-party relations.
THE five principles of peaceful coexistence that Mr. Ji talks about were originally formulated by Indian Premier Jawaharlal Nehru and Chinese Premier Chou En-lai. They are the basis for China's relations with its noncommunist neighbors, and the fact that they are invoked in connection with Chinese-Soviet relations shows how determined Peking is that there be nothing special about its ties with Moscow even after relations are normalized.
China's leaders emphasize that an independent foreign policy does not mean an equidistant policy toward Moscow and Washington. They vigorously criticize US policy in places like the Middle East, South Africa, or Central America, just as they do Soviet actions in Afghanistan or Kampuchea.
But as a Chinese source recently said, ''From our viewpoint, it is Soviet military and political actions around our borders that threatens China. In this sense, our relationship with the Soviet Union is not a friendly one.''
In Chinese-American relations, the same source noted, there was no security threat to China posed by the US and the only real obstacle to better relations was Taiwan, ''which is a problem left over from history.''
Moscow may help China to modernize a few of its factories. This is a drop in the bucket compared with the technological and industrial cooperation the West as a whole, and especially the US, can provide China in its drive to achieve economic modernization. China has more than 10,000 students in the US alone - most of them in science and technology. China's leaders have already made their choice as to how they are to achieve modernization, and for all their attempts to stem the ideological and social side-effects of their opening to the West, they insist that the door, once opened, is not going to be closed again.
Whether Toffler's prophecy about the Third Wave is or is not fulfilled, China's billion people and their leaders face a huge and daunting task.
In the long run, the most significant aspect of the Chinese-American relationship will not be actions and attitudes toward the Soviet security threat , but the way in which the Chinese and American peoples interact in the course of China's new long march toward economic modernization and a more abundant and at least somewhat more open society.