The People's Republic of China has decided to move ahead on a long-discussed official visit here by Premier Zhao Ziyang that will symbolize a major improvement in Sino-American relations and pave the way for a possible trip to Peking by President Reagan early next year.

Administration officials said Peking in recent days has initiated discussion of a trip here by Zhao in December or January, with the possibility that the journey might be announced by the two governments even before Foreign Minister Wu Xueqian visits Washington next month.

Prior to the latest contacts, it was believed that Wu's most important mission here would be to lay groundwork and conclude arrangements for a possible later trip by Zhao.

The sources said there was no clear explanation for the speedup in Chinese willingness to proceed on detailed plans for the Zhao trip. There was speculation, though, that Peking has been heartened by advance word of soon-to-be-announced guidelines covering export to China of sensitive and militarily important technology from U.S. firms.

Reagan is scheduled to depart Nov. 2 to visit five Asian countries, including Japan but not China, the other major power in East Asia. Administration officials have taken the position that a top Chinese leader should visit the United States before a U.S. president next goes to China.

If the Zhao trip here is successful, sources said, Reagan is expected to visit Peking, probably next March or April, before the 1984 presidential election campaign in the United States is fully under way.

Zhao's agreement in principle to visit the United States came in discussions with Secretary of State George P. Shultz in Peking last February. But the Chinese heatedly denied a White House statement at the time that Zhao had agreed to travel before the end of 1983, and in fact there was no such precise agreement, U.S. sources said.

The new diplomatic discussions of Zhao's trip began as Secretary of Defense Caspar W. Weinberger prepared to depart today on a 12-day overseas trip that includes five days in China. The Weinberger visit is expected to restore a degree of momentum to Sino-American military relations that have been nearly stagnant in the past two years.

Then-Secretary of State Alexander M. Haig Jr., on a visit to Peking in June, 1981, announced U.S. willingness to sell weapons to China, and he forecast a visit here by a senior Chinese military official to further military cooperation.

But about that time, the Chinese leadership shifted to a new "independent foreign policy" stance, including improved relations with the Soviet Union and imposition of greater distance between Chinese and U.S. policy in some fields. Until now, China has been unwilling to purchase U.S. weapons or send a senior military official here on an official trip.

Sino-American relations have been uneasy and often strained since Reagan came to office with stronger pro-Taiwan sentiments and statements than any of his recent predecessors. Peking considers Taiwan a wayward province that should not be treated as a separate state.

The resulting wariness made it more difficult to resolve other Sino-American differences in economic, political and human rights fields.

Recently some of the gaps have been narrowed or closed. A Sino-American textile pact was signed last month, a civil nuclear pact is under active negotiation and the new U.S. technology guidelines are be announced within a few days.