Senior Chinese officials, in an unusual press conference, announced today that Peking will send a delegation of nuclear experts to the Soviet Union to study nuclear power plants there.

Vice Premier Li Peng, who heads China's nuclear power program, also disclosed that China is cutting back on its anticipated purchases of foreign nuclear equipment and reactors, raising doubts about the likelihood of multibillion-dollar nuclear sales to China by some western companies.

Analysts said the main reason for the cutback was a drop in China's foreign exchange reserves. The drop was caused by indiscriminate spending on imports, construction and bonuses for industrial workers that began in 1984 and continued into 1985, despite government attempts to stop it.

Western diplomats said the disclosures indicated another upgrading in the economic relations between the two Communist powers. The economic ties have improved steadily in recent years despite a longstanding ideological rift and persistent differences over foreign policy.

At the two-hour session, Li said China had changed its practice of trying to import complete nuclear power stations from abroad and would rely more on its own efforts. He said China would decide whether to purchase Soviet nuclear equipment after the study mission made its report. The Soviets' civilian nuclear technology is said to be inferior to that of western nations, but it is cheaper.

The subject of possible cooperation with the Soviet Union in civilian nuclear technology was discussed during the visit last month to China by Soviet First Vice Premier Ivan Arkhipov, according to diplomats.

Today's press conference at the Great Hall of the People was unprecedented in its length, variety of subject matter and willingness of Chinese officials to answer oral questions, according to some western diplomats.

Industry analysts once estimated that China might be able to purchase nuclear equipment and power plants worth a total of $15 billion to $20 billion by the year 2000. But by the middle of last year, foreign analysts had reduced that estimate to between $5 billion to $10 billion. China is one of only a few markets for nuclear sales at a time when most of the world is moving away from nuclear power.

It was not yet clear what the Chinese cutback in nuclear purchases meant for American companies producing nuclear equipment, such as Westinghouse Corp. Until last summer, when a U.S.-Chinese nuclear cooperation agreement was signed, the companies were behind their European counterparts in entering the market.

In two other disclosures at the press conference, senior officials said that China is still uncertain about starting the projected multibillion-dollar Three Gorges dam project on the Yangtze River and that China has decided to abolish the foreign exchange certificates that foreigners are required to use for many of their purchases in China.

Li Peng said the dam proposal, which could become the biggest hydroelectric project in the world, still posed problems. While most Chinese leaders are believed to favor the project, it has encountered opposition from provincial critics who fear its impact on the environment and problems associated with displacing hundreds of thousands of people in one of China's most populous regions.

Foreign exchange certificates were designed to help control the black market in currency and to help the Chinese government monitor the flow of foreign exchange. But the system, introduced in 1980, did little to help in either way and worked to undermine the regular Chinese currency, known as renminbi, or people's money.

Some observers said it took considerable courage on the part of the Chinese government to recognize the failure of the foreign certificate system and to move to abolish it.

This decision, taken together with the nuclear cutback and continuing indecision over the Three Gorges project, appeared to indicate defensiveness on the part of the Chinese government, these observers said.

But it was not evident in the performance of the officials today before reporters.

During the press conference, called to discuss the current five-year economic plan, Li, Vice Premier Yao Yilin and agricultural expert Du Rensheng took more than 20 questions from reporters on subjects ranging from Sino-Soviet relations to grain crops and oil prices. They answered in a relaxed, self-confident manner.

The most closely watched of the three officials was Vice Premier Li. At 56, the Moscow-trained engineer is a prominent figure among the new generation of Chinese leaders now assuming greater responsibility and is widely considered to be a candidate to become China's next premier. The bespectacled, round-faced Li spoke in a soft voice but with an air of competence and confidence.

Li said China would continue its technical cooperation with West German and French companies. But he said China will expand a nuclear power station of its own design in Zhejiang Province while postponing implementation of a plan to build another power station, with foreign assistance, 100 miles outside Shanghai.