Faced with a surplus of 70,000 doctors by 1990, the nation should start slashing medical school enrollments by nearly one-fifth, a major government commission said yesterday.

Otherwise, said Dr. Alvin Tarlov, the group's chairman, billions of dollars will be added to the country's $20 billion-plus annual medical bill, because more doctors will be doing more things to more people, without necessarily improving health.

Having more doctors will also help some people, he said, mainly be providing more services in underserved inner cities and rural areas.

But in the main, the commission concluded, training should be curtailed not only of medical students but also of most specialists, since cardiologists, obstetricians, allergists, eye doctors and most kinds of surgeons will all be "in surplus." In fact, the group said, there will be nearly twice as many cardiologists and neurosurgeons and 45 percent more obstetricians than the country needs.

The study group, the Graduate Medical Education National Advisory Committee, was named by Health, Education and Welfare Secretary Forrest Matthews in 1976 after Congress asked HEW whether the government was overcorrecting what had been a severe doctor shortage in the 1960s and early 1970s.

The group made its report to Patricia Roberts Harris, secretary of what is now the Department of Health and Human Services. But no quick new HHS action was promised, though four years have elapsed since Congress raised the question.

Dr. Henry Foley head of HHS' Health Resources Administration, said "we will look at" the recommendations" for the next several months." However, the Carter administration already has urged Congress to halt all so-called "capitation" grants to help medical schools increase or maintain enrollments.

Congress has not halted them but has whittled them down from a $2,000 per-student high in 1972 to $639, or less than half that in 1972 dollars, this year. Federal appropriations for the purpose have dropped from $116 million in 1979 to $77.7 million this fiscal year, and the House has voted to slash the sum gradually to $29.3 million by 1983.

A Senate bill would maintain spending at about present levels but give the money to schools that stress primary care fields such as family practice and pediatrics, or teach students to practice in poorly served areas, or increase enrollment of badly underrepresented minorities, mainly blacks, Hispanics and Indians.

The commission recognized that underdoctored areas still need more medical manpower, and minorities still need help in becoming doctors.

But it took a revolutionary view on how Americans' medical care should be given. It said training should be maintained at present levels for "physician extenders," mainly nurse-practitioners, nurse-midwives and physicians' assistants.

And it said these nondoctors should be the only professionals to see the patient in 10 to 20 percent of medical visits in many situations, compared with as few as 1 percent of visits today.

There are now 20,000 such physician extenders, but there will be 50,000 by 1990.The commission, Tarlov said, thinks 20 percent of all checkups for pregnant women could be handled by a nurse-practitioner. It thinks similar practitioners could handle 16 percent of fice-based child care and 12 percent of such adult care.

Among its recommendations, the commission said:

In addition to 17 percent cuts in medical school enrollment, there should be sharp new restrictions on entry by graduates of foreign medical schools. These students -- many of them Americans who could not get into American schools and many graduates of new or expanded Caribbean and Mexican schools -- are becoming doctors here at a rate of 3,000 to 4,000 a year.

There should be adjustments in hospitals' and medical centers' residencies and fellowships to decrease the number of most specialists, while increasing the number of those still in short supply -- among them, psychiatrists, emergency doctors and specialists in physical medicine and rehabilitation.

"Health service reimbursement plans" -- the way doctors are paid -- should be adjusted to "help achieve health policy objectives," such as increasing or decreasing the supply of various kinds of physicians.

To sum up, said Tarlov, chairman of the department of medicine at the University of Chicago, "there is still a slight shortage" of doctors today, but by 1990 there will be 536,000, where just 466,000 will be needed.