China significantly softened its terms for a reconciliation with the United States today, asserting for the first time publicly that it is willing to negotiate a time limit for American military sales to Taiwan.

An official news agency commentary cited "the larger interests" of Sino-American cooperation against Soviet advances to explain why Peking softened its demands for an immediate halt to all U.S. arms sales to Taiwan.

The commentary reflects China's generally conciliatory tone since the Reagan administration announced plans three weeks ago to continue supplying Taiwan with F5E jet fighters while withholding more advanced aircraft.

China had urged in harsh terms that Washington cut off all military supplies to Taiwan, which Peking considers a breakaway province. So much as the sale of a single rifle to Taiwan, Peking had warned, could cause a downgrading of relations.

Proposals to set a time limit on U.S. weapons transfers to Taiwan reportedly were raised privately as far back as October, when Foreign Minister Huang Hua met Secretary of State Alexander M. Haig Jr. in Washington.

Although Peking has stressed its "reasonableness" in settling the issue through negotiations, it has never before publicized its willingness to consider a proposal that would allow weapons sales to Taiwan to continue for some time.

Diplomatic analysts believe the offer was broached publicly today to demonstrate China's flexibility and good intentions toward ongoing talks to resolve the issue, which has overshadowed Sino-American relations for 18 months.

The commentary, issued by the official New China News Agency, said the time limit proposal allows Peking to "safeguard its own sovereignty" while giving "due consideration" to the U.S. side to work out its obligations to Taiwan.

When Washington shifted its recognition from Taipei to Peking in 1979, it agreed to call Taiwan part of China but refused to accede to Peking's demands to cut off arms sales to an island that had been a U.S. ally for 30 years.

Instead, the U.S. Congress passed a law known as the Taiwan Relations Act, which permits Washington to provide defensive weapons to Taiwan, whose Nationalist Chinese government split from the mainland after the Communists took over in 1949.

In today's commentary, the news agency said it "is only natural and legitimate for China to declare its resolute opposition" to U.S. arms sales to Taiwan, which "is indisputably an act of infringement on China's sovereignty."

The commentator noted, however, that Peking is "always mindful of the larger interests" and is "willing to negotiate with the United States for an end to the sales within a time limit."

Despite this apparent bow to Washington as an important friend to have against the Soviets, the commentator warned that Peking would not submit to "blackmail" by Americans who think they can trade U.S. help for Chinese acquiescence on Taiwan.

Americans should not mistake China's "desire for better relations with the United States in the interest of global strategy as proof of need for American help" and willingness to compromise on Taiwan, the commentary added.

The U.S. side offered in June to sell China lethal weapons that could be used to bolster Chinese defenses against the 50 Soviet divisions flanking the Sino-Soviet border.

Although China has sent a list of weapons it would like to purchase, it has postponed a visit to Washington by a high-level military delegation to discuss terms of sale.

Chinese officials say they want to buy American weapons, but not as a tradeoff for U.S. weapons sales to Taiwan.

Today's commentary was designed as a response to U.S. Sen. Jesse Helms (R-N.C.), who was quoted by the news agency as saying that he "greatly resents being put in a blackmail position" by China when it comes to U.S. arms sales to Taiwan.

"The Chinese people do value their cordial relations with the American people," the commentary said. "But this does not mean they will cease from exposing those diehards who insist on poking their noses into China's internal affairs and encroaching on its sovereignty."