Workers who pick low-cost health plans with limited coverage could have all their premiums paid by the government, while those who want broader, better coverage would pay more out of pocket under a radical shakeup of the Federal Employe Health Benefits program now under study.

The FEHB covers 9 million civil servants, family members and U.S. retirees, including nearly half the population of metro Washington. It is the largest group health program in the free world. Its premiums and benefits serve as a trend-setter for health and hospital costs in the private sector.

If President Reagan buys the plan and Congress okays it, the government as the employer would drop the present system whereby Uncle Sam pays a percentage of worker health premiums under a complicated formula-averaging method. Instead, all employes would get the same amount of money each pay period to buy whatever health coverage they wanted and could afford.

Although benefits and premiums vary widely among the 126 health insurance plans in the FEHB, the government now pays a maximum of $39.93 toward premiums of workers who chose the high-option family plans, and a maximum of $17.36 for those who chose self-only high option coverage.

In plans available to most federal workers, the high-option family premium that the employe pays ranges from as much as $41 biweekly to as little as $13.65, depending on the benefits offered.

OPM, which has a major say-so in rates and benefits FEHB carriers offer, would become more like a clearing house. The agency, which developed the proposal, still would okay carriers for the FEHB but would leave it up to the carriers to determine premiums and benefits.

Backers of the "free market" approach argue the fixed payment idea would encourage greater diversity among FEHB plans and help people get more for their money by allowing them to shop around for plans that fit into their health needs and incomes. Those who rarely use health insurance would not have to subsidize through higher premiums benefits paid to heavy users.

Opponents surely will argue that the change could wind up costing many workers--especially retirees and those undergoing costly treatment for mental disorders--a lot more money to buy the same protection they now get through FEHB group plans.