Defense Secretary Caspar W. Weinberger has committed himself to fight next year for restoration of the pay raises military people would lose in October under President Reagan's proposed freeze on federal pay, administration officials said yesterday.

This could add back $5 billion to the defense budget in fiscal 1985 and thereby jeopardize one of the main objectives of imposing a pay freeze, holding down federal deficits in the future.

The Pentagon had recommended a 7.6 percent raise for the 2.1 million men and women in uniform for fiscal 1984, which starts Oct. 1, and had assumed it would take an additional raise of 5.5 percent in fiscal 1985 to keep them even with inflation and pay rates in the private economy.

If these assumptions hold, Weinberger this time next year will be fighting for a 13.1 percent raise, costing $4.98 billion at the going rate of $380 million for each percentage point added to the basic pay of the military.

An increase of this size for the military, moreover, would lead to demands for comparable raises for the government's civilian employes.

Weinberger's intentions were made known by officials who declined to be identified for publication.

The implication is that this year's victory by the Office of Management and Budget in reducing the defense buildup was for one year only, and will be fought all over again a year from now.

What Weinberger has said on the record is that he has agreed to reduce the fiscal 1984 Pentagon budget by $11.3 billion in budget authority and $8 billion in actual spending.

Other Pentagon officials have said $4.9 billion of this spending cut will come from deleting pay raises for both uniformed and civilian defense employes; $4.5 billion from lower than anticipated inflation and reduced fuel prices, and $1.9 billion from assorted other cuts.

Weinberger said he can achieve the economies without canceling or stretching out major weapons programs, an assumption challenged by some White House officials. They say inflation is so much higher in the defense industry than the rest of the economy that cuts inevitably will have to be made in hardware accounts to make the promised reductions in the total budget.

Worse than that from the viewpoint of White House officials outside the OMB who want to go full speed ahead on Reagan's rearmament program, the president, by accepting cuts for his fiscal 1984 defense budget before sending it to Congress, has flattened out its spending slope for future years, no matter what Weinberger does next.

They predict that Congress will keep the Pentagon on a lower spending slope from now on, cutting as much as $100 billion from Reagan's original blueprint for the next five years, barring an international crisis.

Two powerful Republican senators on economic matters said over the weekend that Reagan's announced defense cuts are only the beginning.

"We can do better" than the announced $8 billion spending cut for fiscal 1984, Sen. Robert J. Dole (R-Kan.), chairman of the Finance Committee, said on the NBC "Meet the Press" program. "We ought to take a look, perhaps, at some of the weapons systems."

Sen. Pete V. Domenici (R-N.M.), chairman of the Budget Committee, said on the CBS "Face the Nation" television program that he, too, favored looking for places to cut other than pay.

"What concerns me," continued Domenici in discussing Reagan's proposed defense cut, "is that it doesn't do very much to reduce those very large and growing future year military expenditures." He said the Defense Department has to do more to reduce the federal deficit over the long term.

Weinberger this year has been forced to try to ride two horses at once. He will be trying to convince Congress that cutting military pay is a good way to reduce the deficit at the same time he is trying to reassure service personnel that whatever they lose in fiscal 1984 will be restored in fiscal 1985.

He has said that providing such assurance is vital to attract high-quality people to the All Volunteer Force and keep those with salable skills from quitting for higher paying jobs in the civilian market.