Ambassador to China Leonard Woodcock warned yesterday that China might sharply curtail relations with the United States if Republican presidential candidate Ronald Reagan is elected and expands ties with Taiwan.

Summoning a rare press conference in Peking, Woodcock told reporters that recent Chinese assertions that Reagan's policy could "wreck" U.S. China relations must be taken seriously. Woodcock said, "I don't think they lightly use such words."

Speaking out with unusual public forcefulness in the wake of Reagan's attempt on Monday to quell the political and diplomatic uproar over varying pronouncements on China, Woodcock said:

"To endanger the carefully crafted relationship between the People's Republic of China and the United States which is progressing so well and to our mutual benefit is to run the risk of gravely weakening the American international position at a dangerous time."

Woodcock personally negotiated much of the agreement for full normalization of U.S.-China relations that was concluded in January 1979.

Responding to Woodcock's statements upon leaving Los Angeles yesterday, Reagan said, "Oh, look, this issue is over. I meant what I said in the statement [laying out his views on Taiwan] and I think that sometime someone ought to ask, 'isn't it strange I've had the same position for a long, long time and it only just now has become an issue, now that the campaign is under way?'"

In Washington yesterday, Rep. Lester L. Woff (D-N.Y.), chairman of the House Foreign Affairs subcommittee on Asian and Pacific affairs, who was House floor manager for the Taiwan Relations Act, deplored the controversy aroused by Reagan's remarks.

This dispute, Wolff said at a subcommittee hearing, "needlessly confuses the People's Republic of China as to the true intentions of the United States . . ." In addition, Wolff said, it is "confusing the American people as to the real basis of the growing relationship between the United States and China."

On the Republican side, Rep. Joel Pritchard (Wash.) said, "In no way does anyone in my party want to turn the clock back" on U.S. relations with China. "It is too bad," he said that this has occurred, because it is not necessary."

The subcommittee then proceeded with its own inquiry into U.S.-China relations, dominated by a different matter of concern. It heard testimony from four witnesses, including two former U.S. ambassadors, specialists on Soviet affairs, Malcolm Toon and Raymond Garthoff.

Although usually representing markedly different points of view, Toon and Garthoff joined in expressing deep misgivings about the Carter administration's abandonment of an "even-handed" policy toward China and the Soviet Union. All the witnesses were supporters of normalized relations with China, but they strongly questioned the military-supply relationship developing between China and the United States.

Woodcock in his press conference in Peking, said he had summoned reporters at his own initiative, and not at the request of President Carter or the State Department. But there could be no question that his action would have administration approval.

The Carter administration has pounced on Reagan in this dispute as a target of great opportunity. Carter political strataegists see a windfall in Reagan's remarks on China and the attempt by Republican vice presidential candidate George Bush, a former ambassador to China, to explain Reagan's policy during Bush's controversial visit to Peking last week. It gives the Democrats hope for turning the tables on the GOP's attempt to score the Carter administration for ineptitude in foreign affairs.

Reagan, long a supporter of the government on Taiwan (the island bastion of the Chinese Nationalists who were defeated when the communists conquered the mainland in 1949), attempted on Monday to clairify his position. He charged that, in agreeing to downgrade U.S. relations with Taiwan to an "unofficial" level in order to normalize relations with the People's Republic of China, the Carter administration made unwarranted concessions.

Relations with Taiwan should be regarded as "official," Reagan said -- although to the PRC the word "official" means a reversion to an American "two-China" policy. Reagan said that he, unlike Carter, "would not pretend" that the current relationship is "not official," but he finally said Monday that he is satisfied with existing legislation and "i pledge to enforce it."

Jay Mathews of the Washington Post Foreign Service reported from Peking that Woodcock underlined the concern of U.S. diplomats and businessmen about the long-term effects of a Reagan presidency. Woodcock, a political appointee, said the Chinese demonstrated their indignation about Reagan's position by summoning Woodcock to the foreign ministry for a meeting with Zhang Wenjinx, the vice foreign minister overseeing U.S. China ties.

It is "sophomoric," Toon said to engage in "tweaking the bear's nose" when the Soviet Union is "so paranoiac about China" and "perfectly capable of doing something irrational."

Prof. Valadimir Petrof of George Washington University's Institute for Sino-Soviet studies said that, in Soviet perception, "the Chinese are succeeding in achieving their basic foreign policy objective . . . of maintaining a high level of tension between the United States and the Soviet Union.

Banning Garrett of the University of California's Institute of International Studies said the subcommittee should investigate what has been, since 1975, a developing plan in the U.S. government for a far-reaching military relationship with China in an incremental, step-by-step manner."