A long-dormant nuclear cooperation agreement with China apparently has been been rejuvenated by new written assurances from China on its commitment to control the spread of nuclear weapons, according to Senate and administration officials.

The departments of State and Energy and the Arms Control and Disarmament Agency, apparently reacting to the assurances, formally requested Saturday that President Reagan approve the pact for submission to Congress, senior administration officials said.

The assurances also are aimed at easing congressional opposition to the pact, which would establish conditions under which U.S firms could bid to provide nuclear power equipment, including radioactive material, to China.

Public briefings on the pact are expected today and it could be signed this week, officials said.

Reagan has been pushing his reluctant arms experts for some time to accept the agreement, and the departmental requests apparently indicate that their concerns have been met in time for this week's visit to the United States by Chinese President Li Xiannian.

Reagan wants to propose the agreement to Congress this week as evidence of concrete progress in U.S.-Chinese relations and as evidence that he is conducting important U.S. business despite his recent surgery. But officials said the surgery still may delay his consideration of the final detailed adjustments that both sides want, official sources said. Reagan also wants to be confident that he will not be embarrassed by congressional reaction to the pact if it is announced during Li's visit.

The agreement would allow U.S. nuclear power plant companies, hurting from a U.S. market that has ordered no new reactors since 1978, to try to sell equipment to China, which says it will build two or three plants at an estimated budget of $6 billion. Similar pacts are in effect between the United States and many other nations.

The preliminary initialing of the agreement was the high point of Reagan's April 1984 trip to China, but it was never submitted to Congress because of worries in Congress and among Reagan's arms specialists that it did not contain enough safeguards against nuclear proliferation. They said they were not satisfied by Prime Minister Zhao Ziyang's promise during a dinner toast in Peking that China will not help other countries to develop nuclear weapons.

The concerns increased after intelligence reports that Chinese technicians were in Pakistan at a location where U.S. intelligence thought a weapons program could be under way.

A staff aide to Sen. Alan Cranston (D-Calif.), who made those reports public in May 1984, said yesterday that administration officials told Cranston last week that the Chinese experts were no longer in Pakistan, and that China had agreed to U.S. veto rights over its resale of nuclear technology to third countries. Other members of Congress reported similar briefings.

"Cranston asked that that be submitted in writing as part of the treaty [package] and got that assurance in a briefing last week," said the aide, Gerald Warburg. Cranston, the ranking Democrat on the Foreign Relations subcommittee on East Asian and Pacific affairs, will hold hearings on the pact when it is proposed but will not take a position on it until he hears details of the assurances, Warburg said.

The House Foreign Affairs Committee has also scheduled hearings for Thursday if the pact is announced. Congress has 60 days to veto the pact or it goes into effect automatically.

An administration official said the language -- not available yesterday -- was agreed upon in negotiations that began during a trip by chief arms control negotiator Kenneth L. Adelman to China early last July. The purpose of the trip was to renew dialogue on the points raised by Cranston and others, the official said, and not to change the treaty terms. The official said the pact remains fundamentally the same as the one Reagan initialed in China.

China's position essentially was that it could obtain any nuclear equipment it needed from European companies and that it was up to the United States to decide if it wanted China's business on China's terms.

However, China signed agreements later with Brazil and Japan promising to apply safeguards specified by the International Atomic Energy Agency, which include rights to on-site verification of the agreement, on both sales and purchases, a tougher agreement than the one with the United States. That move will allow China to characterize the new U.S. assurances as consistent with existing policy, another official said.

Li arrived at Niagara Falls, N.Y., yesterday on the first leg of his 10-day U.S. visit, and is scheduled to arrive in Washington today. Reagan is to host a state dinner for him Tuesday night and is expected to announce cultural, educational and fishing rights agreements that have already been worked out.