A presidential task force reported yesterday that it is likely the armed forces can achieve their goal of growing by 188,000 men over the next five years without resorting to a draft, provided military pay keeps pace with wages in the civilian sector.

The task force, set up in July, 1981, under Defense Secretary Caspar W. Weinberger, predicted that the manpower objectives could be met even if the Reagan administration's predictions of an improved economy and lower unemployment come true.

But some outside experts, who attributed recent gains in reenlistments, recruitment and the quality of volunteers mainly to the recession, raised questions about that conclusion.

"If they're assuming they can increase the size of the forces with an improving economy, then I would say they're being overly optimistic," said Martin Binkin, a military manpower analyst with the Brookings Institution.

The task force report comes two weeks before the Nov. 2 election and follows by two weeks the sentencing of the first youth found guilty of refusing to register for the draft. Benjamin Sasway, 21, a California college student, was given a 30-month prison term.

Weinberger and other Pentagon officials stressed yesterday that registration is being maintained only as a precaution against a national emergency, not as a prelude to restoring the draft.

At a news conference yesterday, Weinberger acknowledged that the recession was "one factor" but "not a major factor" in improved recruitment during the last 18 months.

Some of the largest enlistment rates are in parts of the country where unemployment is not serious, he said.

Weinberger contended that the rise in enlistments and reenlistments is due also to the fact that "it is again an honor to wear the uniform. . . . There has been quite a change in the way the military is viewed." Under the Reagan administration, the military manpower situation has undergone a dramatic turnaround. According to the task force, the percentage of enlisted men scoring above the national average on the Armed Forces Qualification Test rose from 55 percent in 1981 to 61 percent in 1982.

At the same time, all services have met their recruitment goals, and some areas have put potential enlistees on waiting lists. Pentagon officials report three applications for each vacancy at officer candidate schools.

However, between now and 1987 the number of men between ages 17 and 21 will decline by about 15 percent, while the armed services must grow by nearly 10 percent from a present strength of 2,099,000 to 2,287,000.

The task force's predictions were based on an assumption that women would continue to make up between 9 and 10 percent of the total enlisted persons in the armed forces for the next five years. The number of women in the enlisted forces grew from 1.6 percent to 9 percent between 1972 and 1981.

Other than a shortfall of 16,000 persons in Army enlistments by 1987, which could be combated with bonuses, the task force reported that the overall growth figure of 10 percent could be met.

But it warned that the expansion program could face difficulties if military pay falls behind civilian compensation. After substantial military pay increases in the last two years, Congress has agreed to a 4 percent cap in fiscal 1983, compared to an expected growth in civilian pay of 8 percent.

To some extent, experts said, the heavy reenlistments of the last few months guarantee that the military will have a strong manpower base for the next three or four years. Pentagon officials also expressed satisfaction at improved quality of volunteers.

In the armed forces as a whole, 33 percent of the enlistees are in the top two intelligence categories, slightly below the percentage for the entire youth population.

"The real question is whether the historically high reenlistment rate in the all-volunteer force will continue when the economy really picks up," said Richard Danzig, a former Pentagon official in the Carter administration.

Experts also noted that several problem areas have shown little improvement over the last 18 months. The task force noted that there is a continuing "serious shortage of manpower" in the Army's individual ready reserve, the manpower pool that is to replace units destroyed in a war that breaks out with little or no warning.

Also, blacks continue to make up a disproportionate share of the enlisted forces -- 22 percent of all the services and 33 percent of the Army in a population only 12 percent black.

Weinberger said he views these percentages as a "tribute to the patriotism" of blacks rather than as a problem.

"There is still a social and racial imbalance in the all-volunteer forces," said Brookings' Binkin. "This is not an issue to some people, but it is to others."