President Reagan's January meeting with China's Premier, Zhao Ziyang, dramatized improvements in US-China relations. But the relationship is also moving forward in unpublicized ways.
The latest evidence of this is an unannounced mission to the United States by a four-man team of Chinese defense and technology experts. They have been visiting defense, aircraft, and high-technology companies around the country.
China's defense minister, Zhang Aiping, is expected to visit Washington sometime after President Reagan goes to China in April. It is the defense minister's son, Zhang Pin, identified by the Far Eastern Economic Review as director of the foreign affairs bureau of China's Commission for Defense, Science, Technology, and Industries, who is leading the current mission to the United States.
In a sense, the four-man group can be seen as an advance team, helping to lay the groundwork for the defense minister's visit. But the calls the team has made on a wide variety of American companies reflect the multifaceted nature of the US-China relationship. They indicate China continues to have a strong interest in American high-technology products, such as computers, electronics, and radar.
According to Roger W. Sullivan, executive vice-president of the National Council for United States-China Trade, the US and China are ''now back to the process of normalizing their defense relationship.''
As Mr. Sullivan sees it, the nations are picking up where they left off at the end of 1980, when both sides sought such a normalization through an exchange of high-level visitors.
The low point in defense relations occurred in August 1981, when the deputy chief of staff of the Chinese People's Liberation Army canceled a visit to Washington because of the disagreement over Taiwan.
The two sides made a resumption of such relations possible in 1982 when the US pledged in a joint communique to reduce the quantity of arms sold to Taiwan gradually and over an unspecified period of time.
Last September, Secretary of Defense Caspar W. Weinberger visited Peking and disclosed that the US would be willing to discuss the sale to China of antitank and antiaircraft weapons. But the US Defense Department, as well as the State Department and Chinese Embassy here, appeared reluctant to discuss either the aim or composition of the current Chinese mission to the US.
A Defense Department spokesman said that the group was not here to negotiate arms purchases. It was here as a result of ''follow-on discussions'' agreed to during the Weinberger visit to China, the spokesman said.
A Defense Department official said that the Department did not want to set a precedent of announcing every such visit that occurred because of a concern that it would reduce the two sides' negotiating flexibility.
Other experts said that the Chinese four-man mission was apparently of such a preliminary nature that there was not much that could be said about it in any detail.
On the US side, there may be a reluctance to play up the embryonic US-China defense relationship just as the US attempts to reopen arms control talks with the Soviet Union. On the Chinese side, there may be a reluctance to leave the impression that China is in any way departing from an independent foreign policy.
In addition to possible military cooperation, another form of ''quiet cooperation'' is taking place - in the field of intelligence sharing. Last October, CIA director William J. Casey met with China's foreign minister.
The director of the US Defense Intelligence Agency made a visit to China after Mr. Weinberger's visit. And negotiations continue concerning the possible sale to China of nuclear technology for peaceful uses.
But few experts expect any large-scale arms sales to China to result from a growing defense relationship.
''The Chinese have been fairly consistent over the last couple of years in saying they really are not interested in buying arms from us,'' said one China expert, A. Doak Barnett at the Johns Hopkins University School of Advanced International Studies. ''They are interested in high technology.''
The US Customs Service revealed last month that it had arrested five persons on charges of conspiring to smuggle over $1 billion worth of high technology such as missile guidance and radar jamming equipment to China. The Chinese government denied involvement. The US apparently is not contesting that denial.