Only one out of five American workers who get health insurance through their employers has a choice of more than one plan, according to a new government study that has large implications for the Reagan administration's efforts to restructure the health insurance industry.

The survey, conducted by the National Center for Health Services Research, found that in 1977, employer-provided health insurance policies covered about 62 million workers plus about 78 million members of their families.

But only 11 million of the 62 million workers surveyed were given the choice of a health plan from among several options. The remaining 51 million were offered only one plan.

Of the 11 million workers with a choice, 7 million were offered a prepaid group health plan as an option. Many health experts believe that prepaid plans, or health maintenance organizations, are the most efficient way of providing medical protection and holding down the skyrocketing costs of health care.

Although the numbers reflect the situation in 1977 and the figures might be somewhat higher now, the study is probably the most detailed ever completed. It was based on interviews with people in 14,000 households.

The study comes as the Reagan administration is formulating its "competition plan," which is intended to cut health care costs.

The theory behind the competition concept is that when workers are given a choice of different plans, some with larger benefits and higher costs to get them, others with lower benefits and lower costs, they will choose the one best suited to their needs and will not purchase excessive coverage.

As a result, the theory runs, they will not use their policies frivolously and put an extra, inflationary strain on the health market. Giving workers a wide choice presumably would also cause insurance companies and group health plans to compete for business, reducing costs.

The study shows that workers have few choices among plans and putting the competition concept into effect across the country would take a massive restructuring of the nation's health plan marketing system.

The tremendous scope of such a restructuring--and uncertainty as to the most effective methods of bringing it about--are among the factors that have delayed a final administration decision on going ahead.

The study also shows that despite the attractions of a prepaid health plan, workers with that option don't always choose it. Only 2.4 million of the 7 million workers with the choice chose a prepaid plan over a major medical or other type of plan offered by the employer.

One argument against the competition theory has been that workers invariably will opt for the most expensive plan, giving them broader coverage than necessary, driving up medical costs. However, the study pointed to a substantial minority of cases where that did not appear to be the case.

Among the subscribers with choices, about 5 million chose the most expensive plan but 3.7 million, a significant figure, chose the least expensive. (Others chose intermediate plans.)

According to the survey, the average total premium per worker for all the employer-offered health plans in 1977 was $775 a year. It was higher for those with family benefit coverage and lower for those with individual coverage.

Of the $775, the worker on average contributed $177, the employer the rest. In many plans, the boss paid the entire amount.

In another study, the Congressional Budget Office concluded that beneficial effects of the competition proposals in bringing down the growth of health costs would probably be evident only in the long run.

For the short run, the study concluded, the most effective approach might be to increase patient cost-sharing (for example, having the Medicare patient pay a larger share of hospital costs for the first 30 days) or to use a prospective-payment system to reimburse hospitals, one in which fixed rates were set in advance and the hospital would not be paid more even if its costs turned out to be higher.