Chairman John G. Tower (R-Tex.) of the Senate Armed Services Committee took his first step away from President Reagan's new defense budget yesterday, saying it could safely be cut enough to reduce actual military spending $2 billion next year.

A $2 billion reduction in spending would require Congress to lop about $8 billion off the $258 billion Reagan has requested in spending authority for fiscal 1983. Spending authority is generally not used up all at once, but spread over several years; it thus takes a large cut in authority to produce a small cut in spending.

Tower declined to specify what he would delete from the president's new defense budget to achieve the $2 billion spending reduction. He also stressed that he was speaking only for himself, not staking out a committee position.

But his testimony was a further sign that even Reagan's staunchest allies on defense now feel they must bow, at least a little, to demands for cuts in the military as well as the domestic budget to hold down the likely government deficit in 1983.

Tower conceded there was some fat in the Pentagon budget while appearing as a witness before the Senate Budget Committee, which sets budget targets for the various activities of government, including national defense.

On the basis of his own reading of the Reagan military budget, Tower said, "I could identify at least $2 billion in savings in outlays."

At another point in the hearing, Tower said that he agreed with Chairman Pete V. Domenici (R-N.M.) of the Senate Budget Committee that it was absolutely essential to reduce the fiscal 1983 government deficit from the roughly $100 billion now projected, in order to drive down interest rates.

Tower contended, however, that the big offender has not been the Pentagon but the explosive growth of domestic programs. He said that over the last 21 years, after allowing for inflation, defense has grown only 12 percent, compared with 234 percent for non-defense programs.

In his 15-page prepared statement to the Budget Committee, Tower repeated his belief that any major cuts in the defense budget should be absorbed by reducing the size of Army divisions, the Navy fleet and Air Force wings rather than denying troops new weapons or cutting back on repairs and training.

"My principal concern is that this committee or some prominent coalition might agree to cut defense, with some hoping to reduce the deficit but others actually intending to add money to domestic programs, in which case the defense program would go down and the deficit would remain high--two unacceptable outcomes."

Stressing that he opposes big cuts of the defense budget, Tower said that if they are indeed imposed "the chiefs of staff are united in their view that they would rather have a slightly smaller, fully manned and well-armed force structure than one which is in effect hollow."

He warned that reducing the Army, Navy, Air Force and Marine Corps to absorb cuts would mean taking some units out of their overseas positions. "Thus," said Tower, "in the final analysis reducing force structure means that some foreign policy commitments will not be met." He said he opposed withdrawing troops from Europe in response to reductions in the military efforts of allies there. Withdrawals of American troops, he said, would provoke "dancing in the streets in Moscow." He said that "we are there not just to defend the blue-eyed European" but to provide a forward defense for the United States as well.