Chinese President Li Xiannian, departing today for Canada and the United States, expressed hope for strengthened U.S.-China relations but also touched on what appears to be a new element of friction by condemning the U.S. Congress for accusing China of using forced abortions and sterilizations in its family planning program.

During a brief airport press conference before he departed on the 20-day trip, Li also reiterated Peking's official stance that the issue of Taiwan remains the "largest obstacle in U.S.-China relations." While his comment on Taiwan was a response to a reporter's question, he volunteered his remarks on this week's and previous congressional action on China's family planning and population program.

Li said such congressional actions were "entirely based on fabrication and distortion." He said the latest amendments passed by the House, which accused China of using forced abortions and sterilizations, constituted interference in China's internal affairs and were "absolutely unacceptable" to China. He added, however, that he did not think the House action would affect his visit to Washington, which begins July 22, following a 10-day visit to Canada.

In Canada Li is scheduled to meet with Prime Minister Brian Mulroney. In Washington, he is expected to meet twice with President Reagan and also is scheduled to meet Secretary of State George P. Shultz and Defense Secretary Caspar W. Weinberger.

On Wednesday, the House approved two amendments to the foreign aid bill. Both amendments go further than pending legislation in condemning China for its population control policies.

The first, which passed 289 to 130, accused China of "crimes against humanity" because, the resolution says, it systematically used forced abortions and sterilizations to enforce population control. The amendment would give the president authority to withhold U.S. development funds from the U.N. Fund for Population Activities if the president found that the U.N. program is funding, directly or indirectly, China's population planning programs.

The second, which passed 234 to 189, gives the president authority to deny U.S. funds to organizations or countries that promote abortion as a method of family planning.

Western diplomats said they did not expect either the Taiwan issue or Li's reaction to congressional votes on China's population control policies to detract from positive developments expected during the visit.

Li is expected to sign "three or four" agreements while in Washington, one western diplomat said, including a fishing agreement that would permit Chinese catches in U.S. waters. It is conceivable, this same diplomat said, that a long-stalled U.S.-Chinese agreement on nuclear nonproliferation also would be signed during the visit, despite the complexity and sensitivity of the agreement, initialed last year.

If the Li visit goes well, it could also set the stage for a visit to the United States in 1986 by Communist Party leader Hu Yaobang, the diplomat said. It would be the first such trip for Hu, who is widely considered the successor to replace China's paramount leader, Deng Xiaoping.

U.S. officials have been arguing for some time that the U.S.-Chinese relationship now has enough depth and momentum to absorb friction from disagreements. About 300 delegations of various types and sizes currently move between the two countries each month.

Other friction areas include U.S. restrictions on Chinese textile exports and the failure earlier this spring to reach agreement on a U.S. Navy visit to a Chinese port.

This will be Li's first visit to the United States. A veteran revolutionary whose age is listed as 76, Li is said to have had some doubts about the side effects of China's economic modernization program and its reopening to the West.

In 1983, Li said in an interview that U.S.-Chinese relations were "not close" and that the United States was "still an imperialist country." But the one-time guerrilla leader and economic planner is also known as a "team player" within the Chinese leadership and is expected to encourage U.S. business investments to help the modernization effort.

U.S. officials see his involvement in the process of improving U.S.-Chinese relations as a positive sign. Although Li's position as president is largely ceremonial, he is a member of the Chinese Politburo's leading six-member standing committee. A Chinese official said Li participates in all major foreign policy decisions, and his entourage includes several leading officials, among them Vice Premier Li Peng, the country's top expert on nuclear energy.

Li Xiannian is likely to raise the Taiwan issue, but it is not likely to be a centerpiece of his talks here, diplomats said. China objects to large-scale U.S. arms sales to Taiwan. Since 1979, Washington has recognized Peking as the sole legitimate government of China and a 1982 accord calls for gradual reductions in U.S. arms shipments to Taiwan.

In recent months, China has intensified its efforts to win support for its policy of peaceful reunification with Taiwan through propaganda broadcasts, new proposals and added inducements to defectors and visitors from Taiwan.

Last December in Peking, Deng Xiaoping gave British Prime Minister Margaret Thatcher a message to take to President Reagan, a western diplomat said. The message amounted to a call to the United States to play a more active role in bringing about a peaceful reunification with Taiwan. But the U.S. response was that this would place Washington in the position of appearing to endorse the Chinese Communists.

The diplomat said there was a sense of urgency by some Chinese leaders for reunification partly because Deng, who is 81, "would like to leave as part of his legacy a reunified motherland or at least some demonstrable movement in that direction" and partly because of a fear that younger successors to Taiwan's aging president, Chiang Ching-kuo, will be even less inclined toward reunification.

Despite apparent differences over this issue, the Communist Party leaders in Peking seem to have set reunification as a major task for the 1990s.

In an interview published last month with a Hong Kong magazine, party leader Hu reiterated this aim and said that if Taiwan vehemently opposed it, China would have to consider blockading the island. Hu said China currently does not have the strength to do this, but he said that if China renounced the use of force, it would only serve to make the Taiwanese leadership "all the more free from anxiety."