U.S. Secretary Harold Brown today ended an unprecedented week-long tour of China's armed forces, giving Sino-American relations what American officials call a new military dimension of uncertain but potentially great importance.

Chinese defense officials and military officers were set to see off their American counterparts at the airport here this morning, following yesterday's last exchange of toasts and information. Brown spoke yesterday of "parallel" but not coordinated military efforts, although officials said the relationship "could be affected by the actions of others" -- meaning China's and America's Soviet adversary.

Brown's last official discussions took place yesterday at a base of the East China fleet at Wusong, where Shanghai's Huangpu River meets the mighty Yangtze River near the East China Sea. There Brown complimented the Chinese on their modernization efforts.He also pledged that the U.S. Navy would remain first in the world to help maintain stability in East Asia and elsewhere.

Zheng Guizhong, commander of the East China fleet, told Brown that "our Navy is lagging behind because of our poor ecenomic base." The Communist army that conquered China 30 years ago was a land-based force and most officials in the new Navy had to be transferred from the Army. So, Zheng said, "our naval force is very young."

Although Brown's visit here was scheduled long before the American hostage crisis in Iran and the Soviet invasion of Afghanistan, those new challenges to American and Chinese influence gave the talks a potency that the two sides found difficult to handle. They refrained from talking about alliances -- the Chinese corrected a mistranslation that used the world -- and instead spoke of "common strategic interests."

The trip itself, the first ever by an American defense secretary to the People's Republic of China, gave an unstated warning to other powers that the United States and China might be better prepared to cooperate militarily in the future if their mutual interests were threatened.

The visit also spelled the end for the time being of the Carter administration's so-called evenhanded policy toward Peking and Moscow. No American defense secretary has ever visited the Soviet Union. Brown's aides announced during his visit here approval of the sale of a sophisticated satellite ground station, which officials pointedly said would not be offered to Moscow. The two sides agreed to discuss the sale of other high-technology equipment and to exchange high-level military delegations.

The trip also forged new and deeper personal contacts between Chinese and American military officers -- vitally important in China to any future formal relations and information exchanges between the two military establishments.

The chief U.S. defense attache in Peking, Chinese-speaking Army Col. William Gilliland, accompanied Brown throughout the trip, as did Air Attache Col. William Webb and Naval Attache Capt. Samuel Monk. Along with many top civilian Pentagon officials on the trip were Vice Adm. Thor Hanson, staff director for the Joint Chiefs of Staff, and Air Force Brig. Gen. Carl R. Smith, Brown's military assistant.

At Wusong yesterday, Brown toured a Chinese missile destroyer and a submarine chaser. At lunch he toasted the Chinese "openness and cooperativeness," and gave Zheng and other Chinese naval officers a detailed private briefing on U.S. naval development. He said later in his toast that the United States would remain "the world's most powerful naval force," and be able to exercise its sea power "particularly in the western Pacific."

An American official remarked later about how far Sino-American relations had come when "a secretary of defense could speak of U.S. strength in the western Pacific and not be thought of as threatening to China."

Peking admits its own Navy is only useful in defending its coast and has applauded U.S. moves that could counter the growing naval strength of the Soviets.

One American official said he felt the Chinese and the Americans had been careful not to let the symbolism of the visit run ahead of the extent of military relations either side could expect at this point. Brown received coverage in the Chinese press appropriate for a visiting American defense secretary, but no more that that, the offfical said. Brown said at the end of his luncheon toast yesterday that "We look forward to continuing talks . . . so we can continue our independent and separate but parallel military efforts."

Brown left Shanghai for Tokyo today, where he will brief Japanese officials on his talks in China, before continuing on to the United States.