President Reagan's defense buildup has peaked and is on its way down to a "dangerous level" as a result of the antideficit measure just signed into law, House Armed Services Committee Chairman Les Aspin (D-Wis.) said yesterday.

Aspin predicted that future defense budgets would decline by more than 10 percent a year under the Gramm-Rudman-Hollings balanced-budget bill, adding that the law means the new Pentagon budget coming to Congress next month "will be dead on arrival."

Defense Department spokesman Robert B. Sims said yesterday that the administration has not lowered its sights. Defense Secretary Caspar W. Weinberger is preparing a fiscal 1987 budget that would provide a real increase of 3 percent over the $302 billion that Reagan thought he would get from Congress this fiscal year. Lawmakers, however, are in the process of providing at least $10 billion less than that total.

"What you're seeing is a defense budget going down as fast as it went up, with double-digit negative growth in the future unless Reagan raises taxes," Aspin said. "It's the sort of situation that a secretary of defense might resign over." But the Democrat stopped short of recommending that Weinberger actually step down in protest of the new law, which the defense secretary publicly opposed before its enactment.

"This is serious," said Aspin. "We're fooling around with the national security. The situation on paper of Gramm-Rudman plus no tax increase is taking us to the point where we can't defend the country."

Under the law, the defense secretary has little flexibility in implementing budget reductions. Except for the money needed to pay and care for people in uniform, the military personnel account, Weinberger cannot cut deeper into one budget account than another. For example, if deficit targets required a 10 percent cut in nondefense and defense spending for fiscal 1986, the Pentagon's procurement, research, operations and maintenance, and other accounts all would have to be reduced by 10 percent.

What flexibility Weinberger has would work this way in fiscal '86. Under the Air Force's aircraft procurement account, for instance, Weinberger could spare the F15 fighter program from a Gramm-Rudman cut but would have to make up the amount by reducing funds for another Air Force airplane program, such as the F16 fighter or B1 bomber. Under the law, he cannot use Army and Navy aircraft procurement programs to offset the savings.

This fiscal straitjacket was necessary, Sen. Carl Levin (D-Mich.) said yesterday, to prevent "a massive shift of power to the president" on congressional budget decisions.

What the nation's defenses lose under the Gramm-Rudman law "are less important than reducing the deficit," Levin said. In response to a Pentagon statement Tuesday that lower defense budgets triggered by the law would comfort the Soviets, Levin said, "$200 billion deficits are even greater comfort to the Russians than the cuts which could happen here if we do not act more logically . . . .

"We have not had the guts to do what we should do," Levin said. "I've come to the conclusion after five years of being around here that we're not going to be able to reduce the deficit without an absolutely crunching mechanism."