While the Reagan administration moves to boost the Defense Department's budget, the corner of the Office of Management and Budget responsible for suggesting economies in military and intelligence spending has been withering, losing 30 percent of its staff of analysts in the past 21 months.

Since April, 1981, when 37 analysts worked in OMB's national security division, 10 analysts have left or are leaving this month, according to sources familiar with the division. One supervisor also left, and the promotions that followed his departure left an analyst's job vacant.

OMB spokesman Edwin L. Dale Jr. said he could not supply the personnel or budget figures for the various divisions within the OMB. But a comparison of agency telephone books for April, 1981, and October, 1982, show no comparable reductions in the number of analysts in divisions that review the budgets of domestic agencies. And in some cases the staffs were increased.

Two of the national security analysts retired, one went to a different OMB division, one left to join a Senate subcommittee staff, one took a two-year leave of absence to complete an academic degree, two have been detailed to other agencies and another has left the Washington area. Only one has been replaced, said several sources, all of whom asked to remain anonymous.

Another analyst is expected to leave shortly to take a job with Lockheed Corp. and yet another will be detailed to another agency at about the same time. Sources said that budgetary pressures at the OMB led some supervisors to ask analysts if they could find other places in government to work on a temporary basis.

Hardest hit was the intelligence review staff, which lost four of its six examiners and replaced only one.

"There is turnover at OMB," Dale said last week. "It's quite common for divisions to change by small amounts regularly. There's a lot of ups and downs in a lot of divisions."

A number of people familiar with the national security division give differing explanations for the departures and the empty slots left unfilled. Two years of budget cuts have forced a reduction in OMB personnel that hit the national security staff relatively harder than other divisions because more of its examiners were high-salaried senior executives.

Still, others said dissatisfaction over the analysts' diminished role contributed to many of the departures. One said that the division "is not doing all that much about defense" spending, in part because of a perception that President Reagan doesn't want information about the defense budget from any source except the Defense Department.

Two factors have contributed to that perception among the budget examiners. One is the centralization of communication. In past administrations, one source said, individual analysts could go to the OMB director's office, sit in a "hot seat" and explain their suggested cuts.

Most of the analysts' ideas are now passed on through OMB Associate Director Alton G. Keel Jr., a former Air Force undersecretary and former staff member of the Senate Armed Services Committee.

In addition, some of the division's current and former staff members say they see the cuts as the fulfillment of the philosophy outlined in the 1980 Republican platform, which said:

"The ill-informed, capricious intrusions of the Office of Management and Budget and the Defense Department Office of Program Analysis and Evaluation have brought defense planning full circle to the worst faults . . ." of the years when Robert S. McNamara was secretary of defense.

"The job of a career person here is to provide the policy makers with alternative views" on how best to spend money for weapons, defense personnel and intelligence activities, said one source. "If the policy makers don't want an alternative view, the question arises: 'What can I do that makes sense?' "

As a former analyst put it, "It was time to move on."

According to one administration official, the OMB ideally should have the ability to look at agency budgets in enough depth to generate independent questions about spending priorities. Without that, OMB Director David A. Stockman is limited to a broader "top-line" analysis, for instance contrasting inflation in military construction costs to inflation in overall construction costs.

Even with the early departures, individuals familiar with the area said the national security division still had the manpower to provide some independent assessment of the figures provided by DOD.

But if the cuts go deep enough, the official said, an OMB division can do little more than look at figures and pass them on with almost no analysis. The national security divison hasn't quite reached that point, according to sources, but it is getting there.

"The administration is kind of ignoring this small division, reducing a source of advice for reform," said one person familiar with the office. "If we were stronger, not all the problems involving defense spending would be solved. But now that there's no one around to say to the emperor that he has no clothes, I don't think things are going to get any better."