A year after the normalization of relations, the U.S.-China connection presents a paradox.

Scientific and technical affairs, trade and exchange of all types are steadily growing, and an impressive U.S. delegation to promote further growth left Washington for Peking over the weekend.

On the highly sensitive security front, there is high-level consideration of the first practical Sino-American military cooperation since the communists came to power in Peking.

On the U.S. side, the Soviet invasion of Afghanistan has shattered the previous policy of "evenhandedness" toward the rival centers of world communism, bringing American policy and rhetoric much closer to Peking's longstanding anti-Soviet line.

By all the international form sheets, Peking should be delighted. But in the view of informed U.S. officials, the Chinese are pausing to take stock of what is happening, rather than rushing forward to take advantage of new developments.

It may be that the world is spinning too fast for the taste of the cautious and historically minded Chinese. It may be that the Chinese are determined to maintain their independent position and maneuvering room with other Third World nations. It may be that China is leery of being left out on a limb by an American administration that may be out of office a year from now.

Whatever the causes -- and those as well as others are being examined and cited -- there is no doubt among U.S. officials that relations with Peking are in a delicate stage.

"China does not like to be used tactically," said an official who has had a central role in policy toward that country. "The Chinese are allergic to hyped-up expectations," said another key official.

Their unstated implication was that the United States should be more careful than it has been in avoiding the appearance of playing an anti-Soviet "China card" in relations with Peking.

The most dramatic recent developments were in the security field, which is the most sensitive aspect of the triangular Washington-Peking-Moscow relationship.

A long-planned trip to China by Secretary of Defense Harold Brown was to be the first step toward a full-scale military dialogue between Washington and Peking. The Soviet invasion of Afghanistan, coming eight days before Brown's scheduled departure, suddenly presented the opportunity to shift the discussion from generalities to a very specific common problem, with possibilities for tangible operational cooperation.

Among matters discussed by Brown with the Chinese were U.S. flights over Chinese territory to supply Pakistan with military equipment on an emergency basis. These would be for the delivery of American hardware or even, if circumstances require, for U.S. delivery of Chinese hardware.

The Chinese, according to guarded accounts available here, neither accepted nor rejected such flights, which would be an unprecedented form of cooperation between the United States and a communist country. The Chinese took the position that this, as well as other U.S. suggestions for coordinated assistance to Pakistan, should be judged in the light to Pakistan's own desires, which are being sounded out in Islamabad by visiting Chinese Foreign Minister Huang Hua.

Washington sources will say little or nothing about suggestions that there was discussion in Peking of covert military aid to Islamic insurgents in Afghanistan, who are now officially described by the State Department as "the nationalist forces" in that country. The Chinese are believed to have aided some of the insurgents for months. There is no indication that covert U.S. help has been foreclosed.

A U.S. proposal that Washington and Peking establish a "hot line" for crisis communication, similar to that between Washington and Moscow, was not accepted by the Chinese, although it remains under consideration. Some American officials had expected China would jump at the chance to be accorded equal status with its Soviet rival in this way, but the reaction was much different.

"They didn't understand it," a Washington source said of the Chinese. "They seemed to think it smacks of treating them as a potential enemy, as in the case of Russia."

In general, the Chinese are reported to be leery of permitting the symbols of the Sino-American relationship to get ahead of its substance. There is the suggestion that internal forces in the Asian communist state may be resistant to a sudden embrace with Uncle Sam, the former capitalist enemy.

Peking's "nonparticipation" (in effect, abstention) in United Nations Secuirty Council vote on sanctions against Iran a week ago was a surprise to some Washington officials, in view of China's supportive attitude and behind-the-scenes diplomacy on the hostage issue. That non vote is now seen as China's bid to distance itself from Washington and maintain a clear "anti-imperialist" stand in the Third World.

While these cautionary notes have begun to appear, the substantive side of the Sino-American relationship continues to grow steadily at the end of the first year of normalization. According to the Chinese, 308 official delegations came to this country last year. About 40,000 American tourists and an uncounted number of U.S. delegations visited China.

From China's standpoint, the latest U.S. delegation, headed by White House science adviser Frank Press, is one of the most important. The Press panel, which left Saturday night, is to review and extend Sino-American scientific and technical cooperation, most of which flows west across the Pacific.

Among the 25 participants in this unusually high-powered team are Robert Frosch, administrator of the National Aeronautics and Space Administration; John E. Reinhardt, director of the International Communications Agency; H. William Menard, director of the U.s. Geological Survey; Richard Atkinson, director of the National Science Foundation; Philip Handler, president of the National Academy of Sciences, and other senior officials of government, business and academic life.

The United States made clear during the Brown trip that it was prepared to supply China with more high technology that has potential military, as well as civilian, applications. The recent U.S. turn against the Soviet Union has eliminated any policy requirements for "evenhandedness" in this regard.

The Chinese emphasized, however, that economic growth rather than military is the top priority for their backward economy. Washington anticipates that "dual use" activities, such as a planned U.S. launch of a Chinese communications satellite, will grow slowly rather than dramatically.

Speedy congressional approval of "most favored nation" trade status for China, provision of U.S. Export-Import Bank credits, and the economic interests of both sides are expected to bring a new increase in overall U.s. China trade.This, too, is expected to be slow but steady -- from $1.2 billion in 1978 to about $2 billion in 1979 to an estimated $2.5 billion to $3 billion this year.

There had been speculation of a possible trip to Washington early this year by Chairman Hua Guo feng, China's leader, to give relations a further spurt. But the Chinese informed the Brown delegation that Hua is too busy to come, at least for the first half of the year.

U.S. officials discounted a report from Tokyo that President Carter might visit Peking, in the absence of a Hua visit here, for a meeting on Iran and Afghanistan.