The State Department has decided to ask Congress for $18 million to replace half of the Soviet citizens employed by the U.S. Embassy in Moscow with Americans, official sources said yesterday.

The proposal, to be presented soon after Congress reconvenes following Labor Day, responds to charges that Soviet employes at the embassy may have had a hand in Soviet secret police operations such as spreading chemical dust to track movements of U.S. diplomats.

Former Central Intelligence Agency director Stansfield Turner said this week that Soviet employes of the Moscow embassy were likely to have been involved in spreading the dust.

Turner called employment of Soviet nationals "one of the weaknesses of our embassy" and advocated improving security by replacing them with Americans.

Rep. James A. Courter (R-N.J.) unsuccessfully sought this May to require that all Soviet employes there be removed through an amendment to the State Department authorization bill. The department staved off final passage of the amendment but began intensive study of a replacement program in response to the congressional pressure.

In a letter to President Reagan Thursday, Courter asked him to order an end to employment of all Soviet citizens within a year. The congressman said bugging of typewriters in the U.S. Embassy in Moscow, reported last March, and placement of the "tracking chemicals" where U.S. diplomats would come in contact with them "require regular access and proximity to our diplomatic personnel and their environs."

According to Courter, the embassy in Moscow and its consulate in Leningrad employed 211 Soviet citizens in support and maintenance jobs as of last April. About the same number of Americans are employed there in more senior and substantive positions.

State Department officials made it clear that the replacement program has been drawn up reluctantly, with the belief that there will be many problems.

Officials expressed doubt that eliminating Soviets from the embassy will have a major impact on security problems there, saying that the KGB secret police will still have ample means to spy on U.S. diplomats in the controlled Soviet society.

The cost of replacing the Soviets, and availability of Americans to work in Moscow in menial tasks, some requiring proficiency in Russian, were cited at State as major difficulties.

For about $300,000 yearly, the embassy now employs Soviets who do jobs that would be covered by the $18 million replacement program, according to a State Department official.

The disparity is caused by the requirement for overseas housing, home leaves, full-scale health and pension programs and other costly services for Americans that are not needed for Soviet citizens.

Assuming that Congress is willing to approve the additional funds, it will still be no easy task to recruit Americans to take the jobs, officials said.

Reflecting this view, Rep. Daniel A. Mica (D-Fla.) said in House debate on the Courter amendment May 8 that, according to U.S. intelligence officers, "the types of Americans that we would have to commandeer . . . custodians and janitors and chauffeurs . . . could be more of a target to the Russians than the personnel we already have there, resulting in greater danger."

Most Soviet embassies, including the one here, employ only Soviet citizens, presumably for security reasons.

In a related development, the bargaining agent for U.S. diplomats asked that hardship pay for American personnel in the Soviet Union should be increased because of exposure to the "potentially harmful" chemical tracking agent.

In a letter to the department, the American Foreign Service Association asked that the hardship differential for U.S. personnel in Mosow and Leningrad be raised from the current 20 percent of base pay to 25 percent.

The department called a special briefing Monday for American diplomats who previously worked in Moscow since 1975 to advise them of the possible health risks of the "tracking chemical."