Peking and Washington have learned to make their differences "manageable" during the first four years of the Reagan administration, particularly on the troublesome issue of Taiwan, Chinese Ambassador Zhang Wenjin said in an interview yesterday.

But the veteran diplomat signaled that in one key area, nuclear relations, China has gone as far as it can in reassuring the United States that it will not share nuclear technology applicable to weapons with other nations.

The Reagan administration has delayed taking to Congress an agreement with China on the sale of nuclear technology until it can get further assurances from Peking that it has not been transferring any such technology to Pakistan, which is widely reported to be acquiring the capability to build a nuclear weapon.

"It seems that China should make some further assurance, but we don't see the point . . . . If, after the negotiation is over, you ask for further assurance on some matters, I don't know when it starts and when it ends.

"You can just continue and make the matter more complicated," Zhang said in a definitive statement of the Chinese position on the nuclear deal, which was the centerpiece of President Reagan's trip to China in April.

China has taken several steps to assure the administration that it opposes the spread of nuclear weapons technology, but U.S. officials are said to have intelligence reports of a Chinese-Pakistani technology link. Pakistan's leaders have denied they are building a nuclear weapon.

Once approved by Congress, the agreement could lead to several billion dollars' worth of contracts for American manufacturers of commercial nuclear reactors.

Despite continuing disagreement between Washington and Peking over the nuclear proliferation issue, Ambassador Zhang presented a largely favorable picure of U.S.-Chinese relations.

At a luncheon interview with Washington Post editors and reporters, Zhang, who has been a key actor for the Chinese in the evolution of relations with the United States ever since Henry Kissinger's secret mission to Peking in 1971, was cordial and relaxed when it came to discussing links between the two nations -- in marked contrast with the testy attitude of Chinese officials four years ago when President Reagan first came to power.

"We now have a good understanding of the importance of our relations," Zhang said. "Second, we also understand much better where the difficulties or the differences are."

"Third, we both understand how to handle those differences so that they will not become unmanageable . . . . We've learned to make it manageable."

"Fourth, the whole environment is much better. There's much better understanding between our peoples."

In a brief reference to the triangular Peking-Washington-Moscow relationship, however, Zhang responded pointedly to a question about prospects for renewed U.S.-Soviet talks by urging that they move forward, "but not at the cost of any third country."

"I think our relationship has made progress in the last four years," said Zhang, a bespectacled, silver-haired diplomat and close associate of the late premier Chou En-lai.

In 1980, when Zhang was a central figure in the Foreign Ministry in Peking dealing with U.S. affairs, candidate Ronald Reagan made China an election issue. He questioned the "normalization" of relations that had been undertaken by the Carter administration. Reagan spoke of a need to strengthen official ties with Taiwan, thus implying that he wanted to pursue a "two Chinas" policy despite agreements made by three presidents that the United States would not do this.

This year China was not an election issue, and Peking appeared to be greatly relieved.

In reminiscing about the 1971 Kissinger mission, Zhang recalled that in Pakistan, where he first met Kissinger before the flight on a Pakistani plane to China, Kissinger emerged out of the middle-of-the-night darkness looking "just like he was walking out of a movie picture."

"Dr. Kissinger was a little excited, a little nervous when he was waiting to meet the premier, because the first handshake would be historic," said the Chinese ambassador, describing Kissinger's first meeting in Peking with Chou En-lai.

Speaking of the most dramatic period in U.S.-China relations in the calm tone of an ever unflappable diplomat, Zhang said that it "took great courage" for president Richard Nixon to travel to China at a time when the United States still had full relations with Taiwan.

Zhang said that the Aug. 17, 1982, communique signed by the Reagan administration with China concerning U.S. arms sales to Taiwan made it possible to overcome the "danger" that such arms sales would sabotage the entire relationship.

In the 1982 communique, the United States declared that it intended "gradually to reduce" its sale of arms to Taiwan, leading over a period of time to a final resolution of the arms sales problem. China consistently has opposed U.S. arms sales to Taiwan and has warned from time to time that failure to settle the issue was bound to impair relations seriously.