Defense Secretary Caspar W. Weinberger has notified Congress that the current Pentagon budget contains an extra $2.2 billion even after absorbing the cuts imposed by the Gramm-Rudman-Hollings deficit reduction law.

Weinberger's disclosure in a letter to Congress last month is about to be used as ammunition by senators eager to demonstrate in a debate on defense spending scheduled to begin today that the Pentagon has more than enough money.

"The squirrels in the the Department of Defense are socking away billions to cushion against budget cuts," Sen. Charles E. Grassley (R-Iowa), a member of the Senate Budget Committee, said yesterday.

The Pentagon's admission that it has more money than it needs to finance the military programs approved by Congress for fiscal year 1986, Grassley said, will make it easier for him and his allies to keep the fiscal 1987 ceiling at $295 billion, as recommended by the Budget Committee.

President Reagan last year initially requested $323 billion for defense in fiscal 1986. Congress appropriated $297 billion and then reduced that figure to $286 billion by imposing $11 billion in cuts to fulfill the anticipated requirements of Gramm-Rudman-Hollings.

Weinberger, who protested the deficit reduction cuts at the time, wrote Congress on March 3 that he had found $2.2 billion "in excess of the amount needed to carry out the programs for which the funds were appropriated." The letter, which is required by law, is vague on where the money was lodged although it mentions falling inflation rates and oil prices.

In a plan which Grassley and other senators will challenge, Weinberger in his letter said he intends to pour the $2.2 billion into other Pentagon accounts, including $401 million to finance pay raises for military personnel and $508 million to fund unidentified military programs.

Holding the Pentagon to $295 billion in fiscal 1987, which begins on Oct. 1, is intended to keep the military budget at the fiscal 1986 level after allowing for inflation, in what is known as zero growth. However, several senators, including Sen. Ernest F. Hollings (D-S.C.) and Sen. Dan Quayle (R-Ind.), contend that $295 billion is not enough to keep up with inflation. They plan to press for raising the ceiling by $5 billion or $6 billion.

Grassley said that in addition to the $2.2 billion revealed by Weinberger, there is an extra $5.2 billion in this year's Pentagon budget which could be carried over into fiscal 1987, for a total of $7.4 billion.

"This substantial figure is understandable in the context of the growing mismatch between projected and actual Department of Defense obligations and of the dramatic overfunding for inflation the past five years," Grassley said last night in a letter to his Senate colleagues.

Given this surplus money, Grassley said, the recommended $295 billion "is on the high side" of what is needed to keep the Pentagon at this year's level of funding.

Weinberger and Adm. William J. Crowe Jr., chairman of the Joint Chiefs of Staff, have repeatedly warned Congress that zero growth in the defense budget would not be enough in light of the Soviet threat. Reagan thought he had a commitment from Congress for a 3 percent increase above the inflation rate for fiscal 1987, Weinberger has stressed.

Last May, just before the Senate Armed Services Committee was preparing to cut the fiscal 1986 budget, Weinberger disclosed that he had found $4 billion in the fiscal 1985 budget which could be poured into the 1986 budget to avoid canceling or reducing military programs.

The disclosure enraged several senators and brought charges that the Pentagon was building a big cushion into its budget. The defense secretary denied the charge, declaring that his department had merely informed Congress of the latest projections as soon as they had become available.