The United States and China signed an unusual nuclear-power cooperation agreement yesterday -- the high point of the U.S. visit of Chinese President Li Xiannian.

President Reagan, looking fit but slightly pale and a bit hoarse 10 days after undergoing surgery for intestinal cancer, met with Li for half an hour at the White House, telling him in Chinese that U.S.-Chinese relations should be in the spirit of "hu jing, hu hui: mutual respect, mutual benefit."

The nuclear agreement, one of four trade and educational pacts signed yesterday, was described as "a framework agreement" that sets conditions for permitting U.S. companies to bid for an estimated $6 billion in potential Chinese nuclear power plant construction business.

First initialed 15 months ago when Reagan visited China, the agreement was not submitted to Congress because of concern there and among Reagan's arms control experts that it did not include enough safeguards to prevent China or a third country from reusing or converting U.S. nuclear materials for military purposes.

The pact was signed yesterday by Energy Secretary John S. Herrington and Chinese Vice Premier Li Peng without a single change, but administration officials said China has repeatedly given and demonstrated a firm commitment to abide by U.S. safeguard requirements. Objections raised as recently as Friday by Secretary of Defense Caspar W. Weinberger were talked out and consensus finally reached over the weekend, the officials said.

In a toast to Li at a State Department lunch, Secretary of State George P. Shultz said the pact "has important positive implications for promoting the peaceful uses of nuclear energy as well as strengthening the world nonproliferation regime." Li responded that the agreement "will be confined to peaceful purposes to bring benefit to our people."

White House spokesman Larry Speakes said, "Obviously, we would not be signing it if we weren't satisfied." Some members of Congress have indicated they are still doubtful, however, and hearings on the pact will begin on Thursday.

The agreement goes into effect at the end of 90 legislative days, unless both chambers reject it. Although debate will probably be lively, most observers expect the pact to stand.

A copy of the signed agreement obtained by The Washington Post included the blunt notation that the pact is "between two nuclear-weapon states and that bilateral safeguards [against using the materials in weapons] are not required." Both sides have many other sources of nuclear material.

The key controversial Section 5 is entitled "Retransfers, Storage, Reprocessing, Enrichment, Alteration and No Use for Military Purposes." It commits the two parties to consultations on any plan to enrich, reprocess, alter or move nuclear materials provided by the United States as well as materials created in U.S.-provided facilities, but does not provide a U.S. veto as some critics have said it should.

Instead, it says, "neither party will seek to gain commercial advantage" through delaying its consent.

Energy Department sources said a key part of the administration's presentation to Congress would be a classified summary of a meeting between Li Peng and special U.S. ambassador and nuclear negotiator Richard T. Kennedy in Peking in June. Kennedy was said to have "nailed down" Chinese assurances that they will work to halt the spread of atomic weapons and will abide by all U.S. safeguard requirements.

The sources said Kennedy wrote the summary and "showed it to the Chinese, and they said it's consistent with the way they view their policies." Sen. Alan Cranston (D-Calif.) said he was promised that written assurances of the Chinese position would be included in the nuclear agreement package.

The two presidents met on the South Lawn of the White House in a sun-splashed ceremony that was slightly shorter than usual in deference to Reagan's continued recuperation and to the frail health of Li, who is 76. Each man remained seated as the other spoke, although they stood for both national anthems.

Reagan took the health initiative by supporting Li's elbow as the two leaders moved in and out of ceremonial position.

"By our common opposition to aggression, we are not only enhancing our mutual security but bolstering world peace as well," Reagan said, in an apparent reference to the Soviet Union.

Li, speaking in Chinese, said to Reagan through a translator that he was "very happy to see that you are recovering so fast."

He added, "The new situation demands our fresh efforts and new achievements" because there are both "great potential to be tapped and obstacles and difficulties to be overcome."

Their private session afterward covered a broad range of issues, a senior administration official told reporters later, including the status of Taiwan, the nuclear trade agreement and U.S. trade policies. The sticky issue of cuts in U.S. funding for population control groups working in China did not come up.

The official said that on the Taiwan issue, Reagan told Li that "new ideas have got to come from the Chinese people on both sides of the strait" that separates the island from the mainland. The United States, he said, should not be an intermediary as the the People's Republic has suggested.

Reagan told Li he was "hopeful" that progress could be made in arms talks with the Soviet Union under the new leadership of Mikhail Gorbachev, the official said. Reagan also assured Li that the administration strongly opposes pending legislation, which would slash textile imports, as "excessive protectionism." Textiles are "a serious concern for China" and a key export, the official noted.

Later in the day, Chinese officials in Li's 14-member delegation signed agreements to expand cultural and educational exchanges between China and the United States and to outline the rules under which Chinese vessels will be allowed to fish in U.S. offshore waters.

At a state dinner last night, Reagan announced that the Chinese government had invited Vice President Bush to visit China in October.

During Li's arrival, an estimated 1,500 persons rallied in Lafayette Square across from the White House and then marched to the Chinese embassy to protest China's Taiwan policies.