All government employes and retirees would get a flat amount of money--in voucher form--to buy health insurance under a proposal the Reagan administration will soon send to Congress.

Under the plan, U.S. workers, who now pay from $300 to $1,600 of their own money for health insurance, would get a government voucher to buy their own coverage.

Everyone--employes and retirees--would get the same payment.

Officials say the amount of the voucher (in effect a check for a specific purpose) would be more than enough to cover the full premiums of some basic plans. Any leftover money could be kept by the employe.

Workers who desired more comprehensive coverage, the so-called Cadillac plans, would have to pay any difference between the voucher amount and the total cost.

Currently the government pays part of an employe's or retiree's health plan, but in no case more than 75 percent of the premium and usually much less than that.

The government share of the premium is based on a complicated formula that is supposed to reflect 60 percent of the average premium cost of six of the largest plans in the program.

The Federal Employe Health Benefits Program now has more than 120 participating plans. They range from nationwide giants like Blue Cross-Blue Shield and Aetna to health maintenance organizations and plans backed by unions for members and associate members.

Administration officials believe vouchers would be simpler and would encourage more carriers to offer plans with bargain premiums.

Allowing employes to keep money from vouchers that is in excess of the premium cost probably would encourage some workers to shop for the lowest-priced plans--though not necessarily the biggest bargains in terms of coverage.

While not everyone agrees with the voucher concept, which has been under consideration for more than a year, nearly everybody agrees that the federal health program needs a lot of work.

Administration attempts to hold down costs (by ordering carriers to cut benefits) have met with stiff resistance from some carriers and unions. Court and media fights between them and the Office of Personnel Management have left a lot of wounds that are not covered by insurance.

Premiums have gone up more than 50 percent in two years (administration officials said it would have been worse except for their efforts), and benefits in many plans have been eliminated or reduced.

Even after last October's 4 percent pay raise, many federal workers say that higher health premiums and the new Medicare tax (1.3 percent of salary) have trimmed 1983 take-home below 1982 rates.

Congress would have to approve the voucher plan. It is likely to have a very rough time before the Democratic-controlled House Post Office-Civil Service Committee, which is eager to tangle with OPM Chief Donald Devine over administration-proposed cutbacks in federal benefits.