President Reagan built up such a huge reserve of defense money in his first four years that Pentagon spending will continue to rise significantly for the rest of his second term even if the one-year budget freeze just passed by the Senate becomes law.

The Democratic minority on the Senate Budget Committee estimates that the defense budget has risen so sharply in the last few years that the United States could retain the present spending level until 1993, partly because of the "bow wave" of money appropriated in earlier years and not yet spent.

The fine print in the compromise worked out between Reagan and Senate Majority Leader Robert J. Dole (R-Kan.) shows that new congressional appropriations for defense would rise $54 billion, or 18 percent, between fiscal 1985 and 1988 while actual spending -- counting funds already in the pipeline -- would jump $61 billion, or 24 percent, in that period.

Citing the numbers, Edwin L. Dale Jr., spokesman for the White House Office of Management and Budget, said "this is a slowdown, not a halt . . . ."

Defense Secretary Caspar W. Weinberger, who has been more successful than any previous peacetime defense secretary in building a big base of funding for the Pentagon, now is in position to decide whether to describe the glass as half empty or half full. The Senate compromise preserves the huge spending base in fiscal 1986, adding only enough to offset inflation, and then continues to raise it in fiscal 1987 and 1988 by adding 3 percent annually to cover inflation.

When Weinberger meets with fellow North Atlantic Treaty Organization defense ministers in Brussels later this month, he can state, according to the Budget Committee minority, that the United States has spent $117 billion more than it was obligated to spend for defense since agreeing in 1978 to raise defense spending by 3 percent a year after allowing for inflation.

Unless Congress cuts deeper than it has done under the Reagan-Dole compromise, happy days still are here for the Defense Department and its contractors. Record high peacetime spending will continue because of the peculiar dynamics of the Pentagon budget.

When the Navy, for example, goes to Congress seeking money for an aircraft carrier costing $4 billion, the whole amount is put in the bank that same year and then drawn out year after year until the carrier is launched. All the services have salted billions of dollars away under this system.

The money will be spent from now until after Reagan leaves office.

From fiscal 1982 through 1985, Congress allowed the Army, Navy, Air Force and Marine Corps to put a total of $369 billion in their procurement accounts, $154 billion more than the previous four years, figured in fiscal 1986 dollars. Spending from the procurement account in fiscal 1982 through 1985 totaled $259 billion in fiscal 1986 dollars, meaning $110 billion more was deposited by Congress than was withdrawn by the Pentagon in that four-year period. It is such unspent money that builds up the bow wave.

Also, the Pentagon has been receiving more money to cover inflation than it needed over the last four years. The House Armed Services Committee is preparing a report criticizing the department for holding onto this inflation dividend, estimated at $30 billion to $50 billion for fiscal 1982 through 1985. The committee is expected to come close to the higher estimate in the report, to be issued this week.

To bring future defense spending down significantly, Congress would have to do more than add fewer dollars to the already high pile.

The quick fixes, however, have such dangerous political side effects that lawmakers are reluctant to try them. One remedy is to cancel giant weapons programs outright, throwing people out of work and incurring cancellation charges from the contractor. Another fix is to reduce the Pentagon payroll of 2.1 million men and women in uniform and another 1 million civilians.

Rather than save money those ways, Congress and the Pentagon in the past have opted for killing new programs in the bud, slowing production of weapons coming out of the factory and cutting operational costs by such steps as fewer flying hours and sailing days.

Stretching out purchases, thus losing savings from mass production while slashing operations, reduces the readiness of the armed forces.

Because the armed services were figuring on a budget increase of at least 5 percent a year for the rest of the Reagan administration, military leaders will be be forced to revamp their five-year plans if the House goes along with the Senate.

The Army is in a manpower crunch, so neither its leaders nor Congress is inclined to reduce the size of the force to save money. This puts such controversial programs as the Divad antiaircraft gun and new helicopter programs on the budget-cutters' chopping block, along with stretchouts in current programs.

The Navy has salted away most of the money needed to build its 600-ship fleet, but will come under increasing pressure from congressional and administration budget-cutters to consider building a mix of nuclear-powered and cheaper conventional submarines, reconsider planned purchases of improved A6 medium bombers and F16 "aggressor" aircraft and slow the tempo of its operations.

The Air Force, also running short of manpower to operate such new weaponry as cruise missiles going to Europe, is trying to build two bombers at once, the Rockwell B1 and Northrop Stealth "Flying Wing." The Stealth is running far above cost estimates, according to Air Force sources, making it a candidate for stretchout.

The Marine Corps is in the midst of its biggest weapons-buying program and will have to slow or kill some programs to fit under the lowered budget ceilings.