Gen. John W. Vessey Jr., chairman of the Joint Chiefs of Staff, flies to Peking next week for the first visit to China of such a high-ranking military official in more than three decades. The visit is seen here as an indication that despite continuing disagreement between the United States and China on a number of issues, relations between the two nations are moving forward steadily.

The Defense Department announced yesterday that Vessey's stay in China will extend for a full week, from Jan. 12 to 19. Pentagon officials said that while they did not expect major results from the trip, it was an important part of what one official described as a longstanding attempt to build "a steady, enduring, normal relationship" between the military establishments of the two nations.

Other officials said, meanwhile, that China and the United States were continuing to discuss the possibility of U.S. Navy ships making their first visits to Chinese ports since the 1949 Communist takeover. One official said that talks on this subject were "moving along very rapidly" and that the first such visit might take place this spring. The official said that it had yet to be decided which port, or ports, the Navy would visit.

A State Department official cautioned that the issue of port calls had first been raised several years ago and that the Chinese might in the end balk at the idea or try to use their approval of such calls as a bargaining chip to trade off for concessions from the United States on other issues.

If port visits do materialize, their symbolism will be enhanced by the fact that European powers used warships to force the Chinese to open their ports in the 19th century. In some quarters in China, there may still be a sensitivity to the sight of foreign warships entering Chinese ports, although several West European nations as well as Canada have made port calls with their navies in recent years.

Defense officials described the projected port calls as highly symbolic -- "a chance to show the flag," in the words of one official. They warned, however, against speculation that such port visits eventually might lead to joint U.S.-Chinese military exercises, calling that far off the mark.

While Chinese leaders spoke in the late l97Os of the need for an anti-Soviet united front, or even some kind of an alliance, they have shifted their foreign policy in recent years until there is no longer any such talk. Chinese officials these days are sensitive to any suggestion that they might be entering a military alliance with the United States.

Partly because of such Chinese sensitivities, the United States has been low key in its announcements concerning cooperation between the two nations' defense establishments. The days when U.S. officials talked about possible "strategic cooperation" between the United States and China appear to be over.

Defense Department officials have made it clear, however, that the United States does, in their view, have an interest in helping to modernize the Chinese defense establishment. According to the Pentagon, the gap between the military capabilities of the Soviet Union and China has increased steadily in favor of the Soviets in recent years. Defense officials say the United States must work to prevent China from falling so far behind the Soviets that Moscow could intimidate Peking.

"We do not seek to build China up into a major power but into a stable, modernizing force for peace," one official said.

In testimony before the House Committee on Foreign Affairs last June, James A. Kelly, deputy assistant secretary of defense, said that a more forthcoming U.S. policy on defense cooperation "could enchance China's perceptions of the long-term value of a friendly relationship with the West, reducing the risk of eventual confrontation with China as well as favorably affecting the near-term global balance of forces."

Kelly said that "an enduring, friendly but not allied relationship with China . . . will enable us to take complementary actions with the Chinese when our common interests are challenged."

A major turning point appeared to have been reached when Secretary of Defense Caspar W. Weinberger visited China in September l983. A number of high-level contacts have occurred since then, including the dispatch last June of the highest level Chinese military delegation to visit the United States. China's Army chief of staff, Yang Dezhi, is expected here about the middle of this year. Also expected this year is the commander of the Chinese Navy, Liu Huaqing. As part of the increasing contacts, the two sides have exchanged training and logistics delegations.

Last June, at the end of a visit to Washington of a high-level Chinese delegation headed by Defense Minister Zhang Aiping, the Defense Department disclosed that it had approved in principle the sale to China of antiaircraft weapons and TOW antitank missiles. These are considered weapons that could be used for defense against the Soviets while not threatening the Nationalist Chinese government on Taiwan.

But actual sales of defense equipment to the Chinese are materializing more slowly than some observers had expected. Although the Chinese have shown interest in coproduction of the wire-guided TOW missiles, they have yet to make clear whether they will go through with a TOW purchase. Last year, the Chinese made their first significant purchase from a U.S. company, acquiring 24 Sikorsky Blackhawk helicopters. Defense Department officials say these are unarmed, transport helicopters for use only in noncombat situations.

"An arms sales relationship takes considerable time -- months and months -- to develop," a Defense Department official said, noting that the Sikorsky deal took more than a year to negotiate.

No one seems to expect the Chinese to make any major arms purchases any time soon. As Defense Minister Zhang explained China's policy in l983, his nation and its military forces are simply too large to expect to meet their equipment needs through purchases abroad.

And, as a recently published book, "Chinese Defense Policy," edited by Gerald Segal and William T. Tow, points out, a major problem has been the high cost of weapons systems produced by the United States and other western nations. The Chinese have given defense allocations a lower priority than economic development.

Despite obvious limitations, however, U.S.-Chinese defense relations have come a long way from what they once were. As Deputy Assistant Secretary of Defense Kelly noted in his House testimony, "We began with limited knowledge of each other's defense establishments and with a legacy of mutual suspicion from almost 30 years of considering one another as enemies."

But visits to China such as one by U.S. Navy Secretary John F. Lehman Jr. last August and the projected trip by Gen. Vessey indicate that the relationship is currently on track.

Some sensitive elements of the defense relationship are probably more advanced than is publicly known. The Far Eastern Economic Review reported last summer that China secretly had sold to the U.S. Air Force half a squadron of its MiG21 "look-alike" fighters, which painted with Soviet colors, are used as "aggressor" aircraft in a training program at a Nevada air base.