President Reagan today will propose a military budget of $277.5 billion for fiscal 1986, about 13 percent more than the Defense Department will spend this year.

The budget would push the Reagan administration's military buildup over the $1 trillion mark, continuing a series of steady increases that began four years ago. Congress has approved more than 96 percent of the administration's defense-budget requests, but the new proposal is likely to provoke an acrimonious debate on Capitol Hill.

Critics charge that the increase is insupportable in the face of mounting budget deficits and proposed domestic-spending cuts, but Reagan, who Defense Secretary Caspar W. Weinberger said spent "hours" studying the Pentagon budget, has said this level of spending is essential for national security.

The budget follows a pattern of the past four years by seeking large increases in weapons procurement, where spending would rise 19 percent, and smaller growth in personnel and in operations and maintenance, which would increase 7 percent. Critics have accused the administration of consistently underfunding those "readiness" accounts, although procurement includes ammunition and spare parts that contribute to readiness.

Much of the weapons procurement money would go toward strategic nuclear programs, the centerpiece of the administration's military buildup. Reagan has asked for $4 billion for 48 MX missiles, $6.2 billion for 48 B1 bombers and $4.7 billion for a Trident submarine and Trident missiles.

Weinberger criticized the "naive talk of unilaterally canceling the MX," which he said is needed for leverage in arms negotiations.

Spending on research and development of new weapons also would increase sharply, from $27.8 billion to $34 billion, or by 22 percent. That jump is fueled by an increase in spending on the Strategic Defense Initiative, dubbed "Star Wars," which would rise from $1.4 billion to $3.7 billion, with increases projected to $4.9 billion in fiscal 1987 and $6.2 billion in fiscal 1988.

The administration's plan calls for spending $31 billion on missile defense research during the next five years.

The Pentagon has tried to deflect members of Congress seeking savings in the defense budget as they wrestle with record federal budget deficits.

"Clearly, defense has not been a major contributor to the rising deficits," the Pentagon said in a statement, "and it is equally valid to note that further defense cuts, which risk weakening our military capability, will not alter in any meaningful way the future federal deficit picture."

The request calls for an increase in budget authority from $284.7 billion this year to $313.7 billion, a 10 percent increase. The Pentagon said that, after inflation, this amounts to 5.9 percent "real" growth.

Budget authority is the money appropriated by Congress to be spent next year or in future years. Outlays, which would rise from $246.3 billion to $277.5 billion, are what the Pentagon expects to spend during the year. Because of the Pentagon's recent emphasis on big-ticket items, such as ships and planes, which take several years to build, more and more of its outlays come from authority approved in previous years. In fiscal 1986, 38 percent of Pentagon spending, or more than $106 billion, would come from previous authority, which are now beyond congressional control.

Weinberger argued in his budget statement that defense spending is "unique" because it is "determined solely by factors external to our nation."

"Unfortunately, there is scant evidence that the diverse threats to world peace and U.S. interests abroad will subside," the budget states. "Moscow's current and past behavior justify no optimistic assumptions about its future intentions."

Weinberger nonetheless held out the prospect, as he did last year, of a tapering off in military-budget growth if Congress funds the Pentagon as the president requests this year.

"We will progress toward a time, only a few years from now, when we can maintain adequate security without substantial increases in defense spending," he said.

But the Pentagon five-year plan shows no slowing soon. Budget authority requests would increase from $313.7 next year to $354 billion in fiscal 1987 and $401.6 billion the following year, with a smaller increase to $438.8 billion in fiscal 1989.

Weinberger said the fiscal 1986 budget request is $36 billion less than he originally planned to seek. That reduction, accepted by the president in two stages last year, meant that the services will not be able to buy 72 fighter jets, four support ships and hundreds of missiles and trucks that they had wanted.

Even with those reductions, however, the Pentagon managed to increase the number of planes and ships it plans to buy in many categories. For example, under the Reagan request, the Marines would buy 46 AV8B Harrier jump jets, up from this year's 32. The Navy would buy 12 EA6B Prowlers, up from six. The Air Force would buy 180 F16s, up from 150; 48 F15s, up from 42; eight U2 spy planes, up from four, and 16 C5 giant cargo planes, up from eight.

Procurement quantities would hold steady for the Army's Apache helicopter, at 144, and the Navy and Marine F18 fighter jet, at 84. Like last year, the Navy would buy three Aegis cruisers, for a little less than $1 billion each, and four nuclear attack submarines, at $700 million apiece.

Weinberger seeks more than $400 million for each of two programs he has delayed and threatened to cancel because of cost overruns and design problems: the Air Force's AMRAAM air-to-air missile and the Army's DIVAD antiaircraft gun. He said in a news conference Saturday that those funding requests reflect the difficulty of canceling programs in midstream and the need for more careful drafting of contracts.

The Pentagon also would provide steep increases in business for Hughes Aircraft Co.'s missile division, which had to stop work for several months last year when military inspectors found serious quality deficiencies in several missiles. The budget calls for 25,000 Hughes TOW 2 antitank missiles, up from 16,000, and 5,000 Maverick missiles, up from about 3,000.

For the first time in many years, the Army won nearly as large an increase as any service, 5.7 percent after inflation. The Air Force budget would grow 5.8 percent, while the Navy and Marine Corps would increase by 4.5 percent -- which a Navy spokesman attributed to better management of ship construction.

The Army is seeking no increases in its active-duty strength of 781,000 men and women, while the Navy and Air Force plan to increase the size of their forces: the Navy by 14,000, to 586,000, and the Air Force by 10,000, to 612,000. The Marines are seeking an increase of 1,000, to 199,000. The Pentagon said the increases, which Congress previously rejected, are needed to operate the ships and weapons coming off the production line.

The Defense Department also seeks to add 19,000 civilian employes, bringing its civilian work force to more than 1.1 million.

Overall, the Pentagon estimates that it will spend $12 billion less in fiscal 1985 than it originally predicted.

Weinberger renewed his call for chemical-weapon production, which Congress has refused to fund since 1969.