The United States not only would be better off militarily but could save as much as $20 billion by approving a new arms control agreement negotiated with the Soviets, Rep. Les Aspin (D-Wis.) said yesterday.

The 26-page study by Aspin, a gad-fly on the House and Armed Services Committee, is part of the counterattack against those who have been warning that President Carter is about to send the Senate a treaty that would make the United States inferior to the Soviet Union in strategic nuclear weapons.

Called SALT II for strategic arms limitation talks two, the past two being negotiated would limit the number of long-range missiles and bombers each side could deploy. The idea is to make Washington and Moscow feel that they could safely stop piling up more and more nuclear weapons - that the agreed upon limits strike an acceptable balance of terror.

"Without SALT II," Aspin said in his study released yesterday, the current arms race between the United States and Soviet Union will accelerate, forcing an extra U.S. expenditure of $20 billion that "we wouldn't have to spend if the SALT accord is ratified."

Even after spending that $20 billion, Aspin contended, the United States would still be worse off comparatively because the Soviet Union, without the brake of SALT II, would most likely rush to do what it does best in strategic weaponry - produce a lot of giant missiles.

All during the missile race with the Soviet Union, the United States, rather than opt for brute force, has stressed accuracy and sophistication, such as dividing one big warhead into several smaller ones to gang-tackle an enemy missile field.

Noting these rival approaches, Aspin argued that even if the SALT II does not stop the nuclear arms race dead, "it will at least put an end to the quantiative race and confine the competition to qualify - the very area that is our strong suit.

"But if we reject the treaty and resume the quantitative race, "we are entering a race in which we are already behind - surely a very curious route to choose."

Reducing his argument to specific numbers that Aspin said had passed muster when reviewed by experts, he predicted this is what would happen to the relative strengths of the United States and Soviet Union between now and 1985 with and without the SALT II agreement:

Missiles and bombers. The United States currently has 83 per cent of the Soviet total. This figure would rise to 91 per cent under a SALT II agreement; fall to 68 per cent without one.

Nuclear warheads. The United States would continue to have 29 per cent more warheads than the Soviets under SALT II but would end up with 7 per cent fewer warheads than the Soviets without the agreement.

Nuclear explosive power (megatonnage). The United States has 29 per cent of Soviet total megatonnage and would remain at this disadvantage under SALT II. Without SALT II, the disadvantage would fall further to 24 per cent of the Soviet megatonnage.

Throwweight. A measure of how heavy a nuclear bomb a missile or bomber can carry. The United States would have 51 per cent of Russia's weight-lifting ability under SALT II and 37 per cent without the agreement.

"Without SALT II," Aspin said, "it would take approximately $20 billion just to bring each one of those percentages back up where they would be if we ratified SALT II." Much of the money, he predicted, would go for building more bomber versions of the Air Force F-111 fighter-bomber and buying additional Minuteman III intercontinental ballistic missiles.

Arguments over the likely impact of the SALT II limits have been handicapped by the fact that there are no official U.S. figures on the proposed SALT II limits. Backers and critics both have been using unofficial figures in the debate.

Aspin, in yesterday's study, said the SALT-II called for limiting the United States and the Soviet Union to a total nuclear offensive force of between 2,100 and 2,250 missiles and bombers.

Without that limitation, Aspin estimated, the 1977 total respective offenses of 2,059 nuclear delivey vehicles for the United States and 2,480 vehicles for the Soviet Union would change dramatically. He estimated the U.S. offense would remain at 2,059 vehicles while the Soviets' would rise to as many as 4,372 nuclear delivery vehicles by 1985.

While Aspin warned against rejecting SALT II, a senior colleague on the House Armed Services Committee, Rep. Samuel S. Stratton (D-N.Y.) warned last week that the proposed SALT II agreement threatens to freeze "into permanency this dangerous imbalance between ourselves and the Soviet - "