The Fairfax County Board of Supervisors decided yesterday to cut the salaries of police detectives by 5 per cent in an effort to distribute benefits more equally among county employees.

Detectives, who wear plain clothes and concentrate on investigating crimes, are now paid 5 per cent more than regular, uniformed police. The 5 per cent added pay no longer will be paid to police assigned as investigators in the future. Those already so assigned will continue to receive the extra pay for three years.

"There are going to be a lot of men upset about this when they find out," said Ronald Yeager, an informal spokesman for the county's 83 police investigators. "The county wants the best it can get, and if you're getting it, you should apy a little more for it."

He said the cut will hurt the investigators' morale "at the very least. There will be a lot of holerin' going on."

Supervisor James M. Scott (D-Providence), who voted for the cut, said, "we are much closer to a situation where we are not providing unusually high benefits to one employee group that other employees don't get."

Last week the Board voted to continue a clothing allowance for investigators of nearly $400 a year. The investigators alos get $500 to start their plain clothes wardrobes when they are first assigned to the work. The Board has also decided to continue overtime pay for police but to eliminate free life insurance for them. Henceforth police will be covered by the same policy as other county employees and will have to make contributions to their coverage.

Benefits including the extra 5 per cent for detectives and the free insurance had been negotiated between the supervisors and police employee groups before collective bargaining between local governments and employee unions in Virginia was struck down by the state Supreme Court last January.

Police investigators in Montgomery and Prince George's counties are not paid salary differentials.

Voting with Scott yesterday to eliminate the added pay were Supervisors Marie B. Travesky (R-Springfield). Audrey Moore (D-Annandale) and John P. Shacochis (R-Dranesville). Voting against were Warren I. Cikins (D-Mount Vernon) and Joseph Alexander (D-Lee). Martha V. Pennino (D-Centreville) and Board Chairman John F. Herrity (R) abstained, while Alan H. Magazine (D-Mason) was absent.