Washington — As President Reagan leaves Washington today on the first leg of his pilgrimage to China, administration officials are carefully stressing that the United States is not playing the ''China card.''
Because US relations with Moscow are chilly, while its ties with Peking are conspicuously expanding, some observers speculate that the administration seeks to build up the People's Republic of China as a military counterweight to the Soviet Union. They cite such developments as the proposed sale of US defensive weapons to China, the supply of sophisticated technology, and possible participation of a Chinese astronaut in the American space program.
There is little doubt that the administration views the forthcoming presidential trip in global and strategic as well as bilateral terms. A paramount goal of the visit is to encourage China's modernization and help bring it into the wider East Asian-Pacific community. This in itself has strategic implications as the Soviets expand their military presence in the Far East.
Asked about selling arms to a nuclear communist country, Secretary of State George Shultz on Wednesday pointed to China's security problems: its long frontier with the Soviet Union; a Soviet-backed Vietnam on another border, occupying Kampuchea and threatening Thailand; and Soviet actions in Afghanistan.
''On issues of that kind, we share their concern and so (the sale of arms) makes sense,'' he told a press briefing.
Comments by other top US officials also indicate that Moscow is not far from their thoughts as the President travels to Peking. ''The fact of a strong US-China relationship, that this trip both represents and helps to reinforce, is something that is a discouragement to Soviet acts of adventurism, and in that way, contributes in the long run to healthier and more stable East-West relations,'' one official said recently.
But administration policymakers insist that the US is seeking a relationship with China in its own right and does not wish to roil US-Soviet ties by playing off the Chinese against the Russians. Moreover, the Chinese themselves are pursuing an independent, nonaligned policy, despite their desire for closer Sino-US ties.
The purpose of the presidential trip is not to make the Soviets concerned, say US officials. It is dictated by the need to to establish stable ties with a potentially powerful nation and by the growing importance of Asia and the Pacific region in US policy.
To underscore this point, administration officials note that last year US trade with the Pacific Basin for the first time exceeded US trade with Europe. Now that China is modernizing its economy, they say, the potential for US business is incalculable.
Significantly, the presidential trip comes at a time when China has shifted its foreign policy. Preoccupied with internal economic development and desirous of world stability, Peking has abandoned excoriating the Soviet Union and is seeking to improve ties with both superpowers. It now backs US-Soviet nuclear arms control.
''Several years ago the Chinese were stressing the need to stand up to Soviet hegemony and complaining about arms control,'' says Zbigniew Brzezinski, a former national-security adviser, who met with Chinese leaders earlier this year. Now ''they are talking about the American buildup fueling international tensions.''
As for their own defense buildup, the Chinese clearly are interested in military cooperation with the US. Defense Secretary Caspar Weinberger traveled to China in 1983, and the Chinese defense minister is expected to visit the US after the President's trip. Recently a four-man team of Chinese defense and technology experts visited the United States.
But it is not thought that Peking wants large-scale arms sales. It has not made any purchases to date. Rather, it is interested in high technology that would enable it to develop its own defense capability, since it is far behind as a military power when measured against the Soviet Union.
Some experts voice concern about the growing US-China military relationship, however. ''I'm bearish on our posture,'' says A.Doak Barnett, a scholar at the Johns Hopkins University. ''We should not be promoting military sales. The Chinese may be getting serious about working out co-production of antitank weapons. But I would be cautionary, taking things on a case-by-case basis and looking at the repercussions.''
The Soviet Union is, in fact, carefully watching the Reagan visit to China, and according to State Department analysts, has stepped up its anti-Chinese propaganda. The Soviets would be worried about a full-blown US-China military alliance, say the analysts, but are not likely to react to a normal, moderate US supply of defensive weapons to China.
As for the strategic ''triangle'' - the US, USSR, and China - administration officials see it as a given factor in today's big-power diplomacy. ''It's like a three-legged stool,'' one official says. ''When one moves, the others move to balance it off. We're now in a cycle where we're warming up to China, but you can only go so far without damaging relations with the other.''
Some China specialists say the triangular relationship has been overplayed as far as Peking's position is concerned. ''The Chinese have made clear they will not line up with us against the Soviet Union,'' says Dr. Barnett. ''They are trying to reduce tensions with the Soviet Union. There will be no big rapprochement, but they're not making the big noises of several years ago. So I doubt the triangle thing is critical.''
Seweryn Bialer, director of the Research Institute on International Change at Columbia University, suggests that the United States does not in fact have a ''China card,'' since it has limited ability to influence China's attitudes and actions. China, he argues, pursues its own national interests. At the moment, for instance, China is trying to strengthen its position in the triangle by normalizing relations with Moscow, an effort stemming less from US policy than from a need to reduce tensions in order to concentrate on internal development.
''The United States would be well-advised to forget about the 'China card,' '' he writes. ''It should be satisfied that there exists an independent China, which by its very existence, its geographical location, its historical attitude toward Russia, its military power, and its experience with the Soviet leadership provides an important obstacle to the expansionist plan of America's main adversary, the Soviet Union.''