The United States, in a joint communique intended to settle a longstanding dispute with the People's Republic of China, promised yesterday to "reduce gradually" U.S. arms sales to Taiwan, leading in time to "a final resolution."

Officials in both Washington and Peking hailed the accord as a basis for averting a serious setback in their relations, but each side interpreted the document in its own way in keeping with its own interests and internal political problems.

Some members of Congress attacked the joint statement as a duplicitous sellout of Taiwan's interest by President Reagan, who was one of the island's most vocal supporters before coming to the White House. While few expressed enthusiasm for the arrangement, some influential lawmakers backed it as necessary for improvement of relations with Peking.

Sen. S.I. Hayakawa (R-Calif.), a noted professor of semantics before coming to Congress, said that although the document suffers from lack of clarity, "the wonderful thing about language is its ability to mean whatever you want it to mean." In this case, ambiguous language reflects an ambiguous situation, he said, adding that "I think we have to live with this ambiguity for some time to come."

Reagan, in a brief exchange with reporters, said his critics are "not telling the truth" in charging him with abandoning Taiwan.

Later the president telephoned television broadcaster Dan Rather to object to statements about Reagan's policy on the CBS Evening News. "There has been no retreat by me . . . . We will continue to arm Taiwan," Reagan said.

In a written statement, he called the joint communique "a mutually satisfactory means of dealing with the historical question of U.S. arms sales to Taiwan" in a way that will promote further development of Sino-American relations while being "consistent with our obligations to the people of Taiwan."

Administration officials insisted that the limitations on future U.S. arms sales to Taiwan, including the plan for a gradual reduction, were buttressed by a statement from the Chinese that it is "fundamental policy" to resolve China's conflict with Taiwan by peaceful means.

If that policy should change, Assistant Secretary of State John H. Holdridge declared, the United States "would, of course, reexamine our position."

Testifying before a special meeting of the Senate Foreign Relations Committee, Holdridge tended to minimize the importance of the U.S. statement that its arms sale restrictions would lead "over a period of time to a final resolution."

The chief Asian affairs expert of the State Department, who acted as the administration's senior public spokesman, said the U.S. pledges "do not provide either a time frame for reductions of U.S. arms sales or for their termination."

"We will continue to make appropriate arms sales to Taiwan based on our assessments of Taiwan's defense needs," Holdridge added.

Along this line, Holdridge said a new U.S. commitment to additional coproduction of F5E jet fighters for Taiwan will be sent to Capitol Hill before the recess scheduled to begin later this week. The decision to proceed with this arms supply was made by Reagan in January, but formal notification to Congress was withheld in order to facilitate the negotiations with Peking.

The nine-point joint communique released in Washington and Peking early yesterday had been under discussion between the two governments since October and under intensive negotiation since early this year.

The communique, which noted that the Taiwan arms issue was left unsettled in the normalization of Sino-American relations on Jan. 1, 1979, contained language reaffirming mutual respect for sovereignty and territorial integrity and forswearing any U.S. "two-Chinas" policy.

The heart of it was a single paragraph stating that the U.S. government:

* "Does not seek to carry out a long-term policy of arms sales to Taiwan."

* Will not exceed, in its continuing sales to Taiwan, levels of quality and quantity of arms supplied since Sino-American normalization.

* "Intends to reduce gradually its sale of arms to Taiwan, leading over a period of time to a final resolution."

State Department officials said no details had been supplied to Peking about the specific criteria to be used in determining the qualitative and quantitative limits that were announced in general terms. "We're dealing with principles, and there will be flexibility" in their implementation, an official said.

U.S. arms sales to Taiwan in calendar year 1980, which totaled $830 million, will probably be considered the upper limit, officials indicated. They said that dollar amounts, numbers of weapons and technological considerations might all be considered in establishing specific future limits.

The arrangement announced yesterday closely resembled a recommendation to Reagan from then-Secretary of State Alexander M. Haig Jr. in a secret memorandum last Nov. 26 that later appeared in the Los Angeles Times:

"We must recognize that mainland capabilities and intentions do not require a level of U.S. arms sales to Taiwan above the final year of the Carter administration, which provided an unusually high ceiling. We can agree to stay within this level, so long as Peking pursues a peaceful Taiwan policy."

This basic negotiating policy made little headway with the Chinese for months, especially during what a senior U.S. official called a "very tense" period of about six weeks in March and April. There was great concern then that the talks might fail, bringing the threatened retrogression in Sino-American relations and perhaps with it a shift in the international strategic balance.

By the time Vice President Bush went to Peking in early May, in a trip that is given credit for advancing the negotiations significantly, the tension had begun to subside, according to official sources.

The trouble was attributed later to internal maneuvering connected with a sweeping reorganization of the government, finally approved in early May, sponsored by Deng Xiaoping, the most important Chinese leader and a sponsor of improved relations with the United States.

Reagan approved a full restatement of U.S. policy in July, shortly after Haig's resignation. After it was presented in Peking, according to U.S. sources, China dropped its key demand for a specific time limit on arms sales to Taiwan.

The most important issue remaining, a problem until the final stage, was China's strong resistance to an explicit link between its peaceful intentions toward Taiwan and the U.S. policy on arms supply.

On July 14, faced with objections from Taiwan and pressure from U.S. conservatives, the Reagan administration supplied six confidential assurances to Taiwan.

As revealed on the island yesterday, the assurances said, among other things, that the United States would not undertake any mediation role between Peking and Taipei or pressure Taiwan to enter negotiations with Peking, and that it had not agreed to hold "prior consultations" with Peking on Taiwan arms sales.

Administration officials may be counting on these assurances, which were repeated in paraphrase in official statements here yesterday, to help quiet conservative objections to the agreement with Peking.