The most momentous result of Secretary of State Alexander M. Haig Jr.'s just-completed mission to Peking was the revelation that the United States is now prepared to sell arms to China. But officials in Haig's party insisted today that in the long run, strategic collaboration on a broad range of anti-Soviet ventures may be just as important.

According to Haig and to the authorized lexicon of American diplomacy, China remains "a friendly nonaligned country" cooperating with the United States rather than a U.S. "ally." Nonetheless, the extraordinary range and depth of proposed cooperation and interdependence suggest an emerging alliance in all but name.

How far the United States will go with China militarily against the Soviet Union has been a hotly debated question within the American government for several years. The answer, which emerged with increasing clarity on Haig's China trip, could have profound effects.

Haig's revelation that the administration has decided to lift the barrier to arms sales and invite a Chinese military mission next month that apparently will be bringing a shopping list, came as a surprise to reporters accompanying the secretary of state. One reason was that they were told by a top official in a June 5 briefing shortly before leaving Washington that "there have been no decisions of any kind with respect to the provision of arms to the People's Republic of China."

Aboard Haig's plane en route from Peking to Manila today the famous "senior official" who speaks authoritatively on such trips insisted that the earlier statement -- which was his own -- is still true. "We've made no decision on the provision of arms to People's Republic of China, none," he declared.

This is technically correct in the sense of a decision on a specific purchase order, because none has yet been requested. However, it is also true that the decision to entertain China's military requests -- reversing a previous U.S. stand that American arms will not be sold to China -- is of far-reaching importance.

The decision had been many months in the making, and in the opinion of many, might well have been taken by the Democrats had Jimmy Carter been reelected. While it had been simmering in the bureaucracy for a longer time, it is reported to have been taken with relatively little hesitation in about a week of National Security Council deliberations before Haig left Washington.

The Reagan administration has "basically thought through" the question of what arms it will be willing, to sell, reporters were told, but no information was disclosed to the press about the items involved.

The most likely weapons, at least to start, appear to be those that could buttress Chinese defenses against the Soviet divisions on its northern border, perhaps as much as one-fourth of the total Soviet armed forces. China is also interested in modernizing its Air Force, whose 6,000 combat aircraft are largely obsolete.

Giving higher priority to economic development, China recently reduced its defense budget by about 20 percent despite the "strategic imperative" of Soviet military power, and it is uncertain where it would obtain the funds to purchase large amounts of American weaponry.

Officials will not say whether they plan to ask that China become eligible for U.S. military sales credits, a form of aid from which it is apparently now barred by law as a communist country. One possibility for saving the Chinese money is coproduction agreements allowing Peking to build weapons under U.S. license.

Whatever the actual Chinese purchases, it is clear that it will be a long time before they could have major impact on the Sino-Soviet military balance. More important in the short run is the political and symbolic significance of such a sales program.

Not only the Soviet Union but China's noncommunist neighbors will be affected, politically if not militarily. The list of concerned nations includes Japan, the Southeast Asian nations which are currently meeting here in Manila, and India (which is also affected by the new U.S. military buildup of Pakistan, its traditional foe). Several of these friendly countries were not consulted or even informed before the U.S. decision on China arms, despite Reagan administration promises of full consultation on important matters.

The potential for strategic coordination and cooperation in mutual Sino-American interests, which is stressed in official appraisals of the results of Haig's trip, is difficult to judge. There have been hints, but little more, of military cooperation by the two countries in Afghanistan and Cambodia to match the political cooperation that is being freely proclaimed by the U.S. aide.

The improvement of Sino-American cooperation depends on satisfactory managment of the touchy Taiwan issue, which is politically sensitive in both capitals. Avoidance of a breach on this issue is far from assured.

It was clear to Haig, according to the senior official, that the United States will have to proceed with great sensitivity on the Taiwan issue in order to maintain the Washington-Peking relationship. The Chinese apparently talked very bluntly to Haig on this question, especially on proposed U.S. arms sales to Taiwan.

American arms for Taiwan, which Peking considers an errant province, cannot be accepted by the Chinese leadership, Haig was informed. But this has been a longstanding Chinese position.

The sale of major new weapons to Taiwan, especially the sophisticated jet planes sought by the Nationalist Chinese, probably would bring a real danger of a rupture of the Peking-Washington relationship. Paradoxically, however, the newly proposed U.S. arms sales to Peking may increase the pressures on the Reagan administration for countervailing sales to Taiwan.

Despite all of Haig's explanations, Taiwan clearly remains an issue that could bring sudden and devastating reverses to the emerging Sino-American collaboration.

There was evidence of this in the final minutes of Haig's stay in China this morning, when Chinese Deputy Foreign Minister Zhang Wanjin stopped the secretary of state on the airport tarmac to protest vigorously and visibly President Reagan's press-conference statements yesterday pledging, as usual, administration compliance with the U.S. Taiwan Relations Act.