Zhang Wenjin, China's ambassador to the United States and a key actor in the evolution of Chinese relations with this country, will end his tour in Washington within the next few months, according to diplomats here. Zhang is to be replaced by Han Xu, another veteran diplomat who once served as deputy chief of the Chinese liaison office here.

If this change signals anything, diplomats say, it is that U.S.-Chinese relations are on a steady course. Han Xu has been serving as director of American and oceanic affairs in the Ministry of Foreign Affairs in Peking, a position formerly held by Zhang.

Diplomats attributed Zhang's impending departure to his age -- over 70. China has placed increasing stress on promoting younger officials. But some Americans who have observed Zhang's work here say that his deftness in handling difficult issues will be missed.

"Zhang was able to make the points he needed to make without sounding ideological or propagandistic," said one American who helps to organize exchange programs with China.

Han Xu, the courtly, crew-cut vice foreign minister who helped to found China's first mission in Washington in 1973, also has had long experience in working with Americans. As a young man during World War II, he is reported to have served in an ambulance unit that rescued downed American pilots returning from bombing raids over Japan. He participated in the talks that led to the establishment of Sino-American diplomatic relations as well as in the 1982 talks on U.S. arms sales to Taiwan.

Zhang's experience with the United States also dates back to World War II, when he worked as an aide to premier Chou En-lai.

Traveling and speaking widely in the United States, Zhang has presided for two years over an increasingly active embassy with more than 90 staff members and more than 50 support personnel.

Zhang played a key role in helping to arrange for Henry Kissinger's secret mission to Peking in 1971, when Kissinger was president Nixon's national security adviser. In an interview last November, Zhang recalled that in Pakistan, where Kissinger was met before the flight to China, Kissinger emerged out of the darkness looking "just like he was walking out of a movie picture."

In his memoir, "White House Years," Kissinger described Zhang's appearance at their first meeting at a Pakistan airport as follows: "Looking like a Spanish cardinal in an El Greco painting -- though of course in Mao uniform -- [Zhang] combined austere elegance and understated intelligence. His command of English was awe-inspiring. . . . He greeted us as if it were the most natural thing in the world . . .."