As China and the United States prepare for an exchange of high-level visits, Foreign Minister Wu Xueqian today praised the Reagan administration for liberalizing technology transfers to China and welcomed the possibly imminent signing of a nuclear cooperation agreement.

Wu, speaking at a rare news conference, gave a generally upbeat assessment of a relationship badly battered by political and economic issues over the past two years. Diplomats said he sought to set a positive tone for his upcoming trip to New York and Washington and the Sept. 25 visit to Peking by U.S. Defense Secretary Caspar W. Weinberger.

Wu also said he expected to discuss new cultural exchanges during his October talks in Washington.

He said he hopes for better relations between Washington and Peking, but stressed that U.S. arms sales to Taiwan--the central dispute in Sino-American relations--still represent an "obstacle."

On Sino-Soviet relations, Wu said he will confer with Soviet Foreign Minister Andrei Gromyko at the U.N. General Assembly session later this month in New York. He spoke before the Soviet Union canceled Gromyko's trip, citing U.S. breaches of its obligations as the host country.

The meeting would have been the first working session between foreign ministers of the two powers since 1969.

The just-completed talks with a Soviet deputy foreign minister in Peking were conducted in a "calm, reasoning and candid atmosphere," said Wu. He repeated China's "sincere hope" to improve relations with its northern neighbor.

But Wu emphasized that prospects for normalizing relations with Moscow remain blocked by the "real threat" to China's security posed by Moscow's aggressive military stance in Asia.

Peking has urged Moscow to pull back its troops from Afghanistan and from the Sino-Soviet border and cut off aid to Vietnamese forces in Cambodia. For the first time, the official People's Daily today reported a new demand--removal of the medium-range, SS20 missiles the Kremlin has been deploying in Asia.

Asked to discuss the new triangular shape of global politics with Peking at the apex, Wu said his government's dealings with one of the superpowers has nothing to do with its relations with the other.

In his hour-long news conference, Wu seemed intent on staking out China's independence as it readies for a busy diplomatic season. In addition to his travels and Weinberger's visit, Peking will host the third round of normalization talks with the Soviet Union next month.

His conciliatory view of Sino-American relations is consistent with the general mellowing of Chinese policy since the Reagan administration decided in June to permit sales of more sophisticated technology to China.

Although specific purchasing guidelines are still being drafted, the decision shifted China into the category of "friendly, nonallied" nations with the right to acquire higher levels of electronics, computers and know-how--all with military application.

Asked if Chinese leaders would discuss military technology or equipment purchases with Weinberger, Wu said such questions are outside his purview.

Previous bureaucratic delays on Chinese requests for high technology were near the top of Peking's long list of grievances against the United States in two years of intense bickering.

Besides the technology transfer, the near completion of a nuclear cooperation agreement that would open the way for U.S. firms to bid on lucrative contracts to help develop China's atomic energy industry is another example of improved relations.

A U.S. delegation headed by special ambassador for nuclear affairs Richard Kennedy is scheduled to arrive in Peking on Monday to discuss final details, according to diplomats. Wu said he would welcome an early signing of an agreement, which, he added, would include guarantees by Peking not to transfer U.S. nuclear technology to a third nation.

The major stumbling block to an agreement has been China's unwillingness to ensure that nuclear materials and equipment would not be converted to military purposes or resold to a non-nuclear state.

China has refused to sign the international Nuclear Nonproliferation Treaty or to allow inspections of its nuclear facilities by outside organizations. China has had its own nuclear weapons since 1963.

Since the liberalization of technology sales, many irritants that have strained relations have steadily melted away. The dispute over U.S. quotas on Chinese textile imports was solved in a new agreement last month. Peking has muted its criticism of U.S. asylum for a Chinese tennis star, which resulted in the breakoff of official cultural ties.

Wu made plain, however, that the improvements were only small adjustments to a relationship still hampered by U.S. arms sales to Taiwan. He called on Washington to fulfill its August 1981 pledge to phase out weapons supplies to Taipei.