One of the least known of Washington's important people is expected to leave this month after a six-year assignment here. Few have ever heard his name. He works exclusively behind the scenes, and there is probably no one in this country whose innermost secrets the U.S. government would more like to have.

He is Dmitri I. Yakushkin, according to the diplomatic registry one of 10 counselors at the Soviet Embassy. Fluent in English, charming, well-educated and cultured, Yakushkin is head of U.S. operations for the KGB, the Soviet intelligence service.

With the rank of major general and the title of KGB resident in Washington, Yakushkin, 58, is the most powerful KGB officer outside the Soviet Union, according to authoritative U.S. intelligence sources.

Yakushkin's expected return to the Soviet has some U.S. intelligence officials watching expectantly for his replacement and what, in the arcane world of intelligence tea-leaf reading, that choice may foretell about the direction and nature of Soviet intelligence operations. The FBI, charged with counterintelligence operations and accordingly the nation's chief spy-watching agency, is convinced that the Soviets have stepped up their intelligence efforts here recently. Last month Attorney General William French Smith said in a speech that the Soviets have "dramatically" increased their spy work.

"About one third of the Soviet-bloc personnel in the United States assigned to embassies, consulates, and the U.N. or other international organizations are believed to be full-time intelligence officers," Smith said.

It is this group of spies, estimated by one U.S. official at about 500, that Yakushkin heads. His replacement will be carefully selected, according to U.S. officials, and the successor's personality and style of operations will provide a good clue to the future of Soviet-bloc activity here.

Yakushkin, KGB resident here since late March, 1975, does not fit the stereotype of an ill-mannered ideologue with an electric shock machine and poorly fitting suits.

A moderate of refined temperament, he reportedly has voiced concerns about human rights issues and the need for arms control. His interests are broad. With a degree in economic science, he is said to keep up with literature and other cultural developments.

Yakushkin was scheduled to return to the Soviet Union and retirement much earlier last year. But he has been held over here, according to officials, for two reasons: his experience provided continuity as the Soviets dealt with the new Reagan administration, and he is close to Soviet Ambassador Anatoliy F. Dobrynin.

Yakushkin has emerged over the last several years as an extremely important and influential adviser to Dobrynin--a rare if not unique role for a KGB chief in Washington.

Yakushkin's wife, Irina, is a translator, but is also a member of the KGB, as is the chauffeur who drives Yakushkin around Washington, according to American sources. The Yakushkins previously lived at the Irene apartments in Chevy Chase, and recently moved to the new Soviet housing complex at 2601 Tunlaw Rd. NW.

Yakushkin has a son, Dmitri D. Yakushkin, 24, who is a correspondent for the Soviet newspaper Pravda in Moscow. Last August the son visited the United States, but Yakushkin was in Moscow, according to officials.

The father and son did not see each other, the officials said, because the son is for all practical purposes a hostage, kept at home to prevent the father from defecting or doing anything else that the Soviet authorities might disapprove. Father and son are not allowed outside the Soviet Union at the same time.

As the resident here, Yakushkin probably knows more about Soviet intelligence operations--including any sensitive penetrations of U.S. government departments or agencies--than any other person in the KGB. Since he served previously for six years as head of the KGB at the United Nations in New York City, his 12 years of U.S. intelligence operations give him unique knowledge and perspective.

A workaholic, Yakushkin reportedly arrives at the Soviet Embassy about 8 or 9 a.m., and often stays to 8 or 9 p.m. or later. On occasion he spends a weekend with his wife at the Soviet recreational retreat in Maryland.

His direct contacts with others in Washington are held to an absolute minimum. The Czech ambassador, Dr. Jaromir Johanes, is reportedly Yakushkin's best friend outside the embassy network.

Yakushkin's first assignment to the United States was a brief stay as a part of the Soviet delegation to the United Nations in late 1962. In July, 1963, he was assigned to the U.N. delegation as head of the KGB operations. He served there until May, 1969, when he reportedly was put in charge of the Moscow branch of the KGB that deals with intelligence operations in Britain.

There were some indications that Yakushkin was willing to be interviewed by a reporter last month, but through a Soviet Embassy spokesman he eventually declined. The spokesman said neither the embassy nor Yakushkin would have any comment on his work here for the Soviet government.