Defense Secretary Caspar W. Weinberger yesterday announced that the administration was trimming its proposed defense buildup next fiscal year by $8.7 billion, an amount he said was proof that the Pentagon took "seriously" its obligation to help reduce the likely deficit.

But some of the Weinberger cuts had been announced before, others were unspecified, and the initial reaction from Republicans and Democrats in Congress was that further cuts would be needed to make the fiscal 1986 defense budget request salable on Capitol Hill.

Defense spending still would rise at least $15 billion in fiscal 1986, and the Weinberger cuts would leave the administration $25 billion short of its announced goal of halving the deficit to $100 billion in three years.

One feature of the Weinberger spending plan would be to give military people a scheduled 4 percent pay raise this Jan. 1 and a 3 percent raise next July, after which pay would be frozen for 15 months.

House Minority Leader Robert H. Michel (R-Ill.) was said by an aide to be disappointed in the Weinberger plan and to believe it was "far short of what we need" to pass a budget in the House.

"It's going to make our job tougher," said Rep. Dick Cheney (R-Wyo.), a member of the Republican leadership. "If we're not going to go flat out on defense, it's going to make it more difficult to get the domestic cuts we need." President Reagan will propose about $34 billion in domestic spending cuts next year.

New Senate Majority Whip Alan K. Simpson (R-Wyo.) said he was "glad to see some movement on the part of the Pentagon , but I'm waiting for more." Sen. John H. Chafee (R-R.I.), another member of the incoming GOP leadership, said the proposed savings were "minuscule," adding, "It's really not a credible performance."

This comment was echoed anonymously in the White House, where ranking officials conceded last week that Reagan was listening to Weinberger's defense budget recommendations rather than the proposals of Office of Management and Budget Director David A. Stockman.

"Once more we're depending on Congress to rescue us from ourselves," said one administration official.

But Weinberger said in a news conference announcing his "contributions" to deficit reduction yesterday that Stockman's proposals would have "savagely" reduced Defense Department spending authority. He said that any freeze on defense spending would be "extremely dangerous" for the United States.

Stockman had proposed reductions in Pentagon spending of $8 billion in fiscal 1986, $20 billion in 1987 and $30 billion in 1988. While Weinberger in his announcement emphasized that his proposals for 1986 exceeded Stockman's, his plan would trim outlays by about $9 billion in 1987 and $10 billion in 1988 for a total of $28 billion, compared with $58 billion in the Stockman plan.

Even more important is that much of Weinberger's "savings" are in shuffling of military and civilian pay rates from year to year, while Stockman would have limited weapons procurement.

The defense budget averaged an annual after-inflation growth of 9 percent during Reagan's first term. Defense budget authority, the right to obligate money to be spent in future as well as the current year, is projected at $293 billion this year.

Authority would rise to $313.7 billion in fiscal 1986 under the changes announced yesterday by Weinberger, to $362.6 billion in 1987 and $411.5 billion in 1988.

"Obviously, the Congress will cut it," Cheney said.

The Republican congressmen who criticized Weinberger were faithful in their support of Reagan's first-term defense spending increases. Democrats, some of whom opposed these increases, were even blunter in their statements yesterday.

Rep. Thomas J. Downey (D-N.Y.) called the Weinberger proposals "absurd, laughable" and Rep. Joseph P. Addabbo (D-N.Y.), chairman of the House Appropriations subcommittee on defense, said, "They pump up the budget with fat, we make cuts, they take credit for them."

Of the $8.7 billion in reductions announced by Weinberger, $1.1 billion in a civilian pay cut had been included in Stockman's previously proposed $33.6 billion in domestic reductions, according to OMB spokesman Ed Dale.

The Weinberger plan also included $2.5 billion in unidentified program cuts, and nearly $1 billion achieved by adjusting amounts set aside for inflation and likely fuel cost increases. The rest of the "cuts" were in pay, and were achieved in part by increasing pay rates more than previously planned this fiscal year, then freezing them.

"Cap is a hero in the building," said one longtime Pentagon executive. "We'll end up with more money under his strategy but with worse programs at the end because Congress will nickel-and-dime us to death through cuts here and there, stretchouts in big programs, smaller buys of spare parts, delays in maintenance."

Other defense officials doubted that Congress would go along with Weinberger's proposal to raise military pay at the same time civilians would have theirs reduced under the Reagan plan. Pentagon veterans of these budget battles predicted the military and civilians both will end up with raises of about 4 percent in calendar years 1985 and 1986.

On military pay, Reagan has approved the Weinberger recommendation to stick with the 4 percent raise to be added to paychecks as of next Jan. 1.

On top of that 4 percent, the military would receive a 3 percent raise for July, August and September of 1985. They would not get a raise on Oct. 1, 1985, the start of fiscal 1986, but would hang on to their previous raises of 4 percent and 3 percent.

Weinberger would leave the military with a 7 percent raise by the end of this fiscal year, no raise in fiscal 1986. The Pentagon had planned a 4 percent raise this year, then 7.1 percent for fiscal 1986.

The 1 million civilians who work at the Defense Department also would get a raise this January, then see their pay cut 5 percent next summer.

Weinberger served notice at his news conference yesterday that he intends to stick to the defense funding profile sculpted in the White House Rose Garden last March when it comes to fiscal 1987 and 1988, the so-called out-years that Stockman and other budget officials consider key to bringing down the deficit.

Here is the new defense profile recommended by Weinberger and approved by Reagan:

* Fiscal year 1986. There would be $313.7 billion in budget authority (the amount that can be obligated in a year though not necessarily spent); $277.5 billion in actual spending. These are reductions of $11.1 billion in budget authority and $8.7 billion in spending from the Rose Garden levels of $324.8 billion and $286.2 billion. However, the new totals represent increases of $22.6 billion in budget authority and $18.9 billion in spending over fiscal 1985 estimates contained in the administration's midsession spending review.

* Fiscal year 1987. The Defense Department would have $362.6 billion in budget authority and $324.8 billion in spending.

* Fiscal year 1988. The Pentagon would have $411.5 billion in budget authority and $358.8 billion in spending.