Seventy-one Fairfax County police detectives have filed a $500,000 suit in Fairfax County Circuit Court against their police chief and the county government, claiming they are not paid enough for the specialized skills their jobs require.

Fairfax detectives are paid $400 a year more than the county's uniformed officers, the suit alleges, while detectives in Arlington and Alexandria receive about $1,800 a year more than those in uniform.

The suit, filed on Monday, charges that senior county officials have ignored the county's own merit system and personnel rules by failing to pay sufficiently higher salaries to detectives who do more demanding work than uniformed police.

Given the same level of experience, a guy writing parking meter violations is getting the same pay as a guy looking into a homicide," said John J. Geraghty, an Arlington attorney representing the detectives.

Named in the suit are Police Chief Col. Richard A. King, county executive Leonard Whorton, personnel director Dale E. Friesz and the county Board of Supervisors. King, Whorton and Friesz yesterday refused to comment, saying they had not yet seen the court papers.

Geraghty said yesterday that the $500,000 figure represents an estimate of the amount the detectives have been underpaid and will be underpaid until the litigation has run its course.

The detectives have sought higher pay through the County Civil Service Commission since August 1977, according to Geraghty.

They also have sought a system of rankings among detectives - a request blocked by Chief King's recalcitrance, Geraghty charged yesterday.

Whorton, the county executive, said that there are no specialized detective classifications and that, given the same amount of experience, all detectives are paid the same whether they investigate murders or petty theft.

Geraghty said police department, in refusing to give higher pay to skilled investigators, is "saying that it does not need to provide any incentives to its detectives." A system that ranked homicide investigators highest would "give the police something to aspire to," Geraghty said.

Under the existing system, Geraghty said, detectives tell him many policemen do not want to be homicide detectives because the job entails more responsibility and work without a corresponding increase in pay.