Washington — Ronald Reagan may have been the darling of the ''free China,'' pro-Taiwan lobby during the presidential campaign four years ago. But he clearly has advanced the strategic cooperation efforts with the People's Republic of China begun under President Carter.
Following a relatively cool period during the first months of Reagan's term of office, things have picked up considerably. Defense Secretary Caspar W. Weinberger's visit to Peking last September resumed the strategic dialogue and laid the groundwork for providing key military technologies to the Chinese.
That is continuing this week with the visit of Chinese Defense Minister Zhang Aiping. He arrived in Washington over the weekend and will be touring United States military installations as well as meeting with key officials here.
With their conservative constitutency in mind, administration officials stress that the US and the PRC are ''friendly but not allied countries,'' and that the relationship ''will move in measured steps.''
But former national-security adviser Zbigniew Brzezinski calls the two countries ''de facto allies,'' and says: ''There is no doubt that in certain key areas, we and the Chinese are collaborating already.'' Dr. Brzezinski was in China this spring to talk with Chinese leaders, and briefed Reagan administration officials when he returned. He expects to meet with Mr. Zhang this week.
According to Pentagon officials, military training and logistics delegations now are travelling back and forth between the countries. Export licenses have been granted for ''defensive'' weapons like antitank and antiaircraft guns. And the US already has provided computers and radars for Chinese military use.
Meanwhile, strategic cooperation apparently is having a positive effect on US diplomacy in other parts of the region. Chinese officials, at the request of President Reagan when he was in China recently, reportedly have agreed to act as a go-between in reducing tensions between North and South Korea.
''One of our aims in strengthening the defense component of our relations with China is to reinforce these positive trends in (Peking's) foreign policies, '' says James Kelly, deputy assistant secretary of defense for East Asian affairs. ''Over the past several years, China has moderated its foreign policies and demonstrated a real desire to improve state-to-state relations with its Asian neighbors.''
As US-Soviet relations continue to ebb, Washington's warming relationship with Peking also helps offset what American officials see as an increasingly prickly Moscow.
In congressional testimony last week, Mr. Kelly spoke of cooperative efforts ''which will enable us to take complimentary actions with the Chinese when our common interests are challenged.''
But the superpower waltz apparently works both ways. ''If American-Chinese military cooperation poses a threat to the Soviet Union, its friends and allies, '' a commentator in Moscow warned on the eve of Zhang's visit, ''Moscow will undoubtedly find an adequate answer to any menace.''
Soviet and Chinese forces constitute the largest mass of military might faced off across a border. There are about 1 million Chinese soldiers on that front and about half that number of Red Army troops.
But the Soviets are much better equipped. They have nearly five times as many tanks and other armored vehicles, air power roughly 20 years ahead of the Chinese, and vastly superior nuclear forces. Moscow has stationed 135 of its modern, three-warhead SS-20 intermediate-range nuclear missiles in Asia.
For this reason, the US and its European allies want to help beef up Chinese military capabilities. France and Britain have sold helicopters, aircraft engines, artillery and fire-control equipment, and radars to China.
The National Conservative Political Action Committee and other groups on the political right are highly critical of the administration's shift toward the PRC. Without mentioning Taiwan by name, the Defense Department's Kelly sought to counter this concern when he stressed in his testimony that ''the question of potential risk to other friendly states is a prime consideration in the approval of any arms sale to the PRC.''
China has begun to modernize its strategic nuclear forces and last year launched its first ballistic-missile submarine. But experts say there is an uneasy balance in China between improving nuclear forces to deter attack and mass mobilization of China's 4 million-man Army.
''Despite changes in the political leadership, there remain many supporters of the strategic concept that mass manpower is still the primary deterrent,'' states a report by the International Institute for Strategic Studies. Still, there have been clear lessons that vast advantages in ground troops may not suffice in modern combat. The People's Liberation Army suffered heavy casualties and withdrew shortly after it marched into Vietnam in 1979 and faced advanced Soviet weaponry.
The Chinese military also labors under an officer corps still recovering from the excesses of the Cultural Revolution. Reform continues, but this takes time.
National defense for the present remains at the bottom of China's ''four modernizations'' (behind agriculture, industry, and science and technology). Annual defense expenditures have remained at about $8 billion, or about 15 percent of the national budget, since the late 1970s, according to the US Defense Intelligence Agency.
For this reason, as Defense Minister Zhang stressed in a press conference just before he left to visit the US, China will continue to seek advanced technology rather than more expensive modern weaponry.
''The Chinese face enormous needs with limited funds, and they must be careful shoppers,'' says Kelly. ''In most cases, they do not consider foreign procurement of end items to be a viable option. They want the technology with which to manufacture their own weapons.''