President Reagan's trillion-dollar rearmament program bought improvements in some U.S. military capabilities, but in other key defense areas the nation is no better off or lost ground to the Soviet Union, according to a congressional report released yesterday.

The assessment said that record U.S. peacetime defense spending during Reagan's first term did little to alter the strategic balance, chiefly because of a rapid Soviet buildup.

"Some U.S. problems have been mitigated, but many remain and a few are magnified," John M. Collins, defense analyst for the Congressional Research Service, concluded in his report, "U.S.-Soviet Military Balance: 1980-1985."

The report was prepared at the request of several lawmakers with influential roles in national security debates, including Rep. Les Aspin (D-Wis.), chairman of the House Armed Services Committee, and Sens. Sam Nunn (D-Ga.) and John W. Warner (R-Va.), members of the Senate Armed Services Committee.

It comes at a time when Congress is nearing final approval of a $302.5 billion defense authorization bill for fiscal 1986, and the findings are expected to be cited by legislators seeking to boost military spending above the budget's "zero growth" levels.

The 360-page report recommends a "fresh look" at U.S. defense capabilities to assign priorities to critical areas before proceeding with policies and programs established in 1980.

Collins, at a news conference, cited the "very impressive improvements" achieved by the Reagan rearmament program. Among those cited are the strengthening of the U.S. technological base, accelerated modernization of tanks, aircraft and ships and the enhanced combat readiness and sustainability of the armed forces.

But, Collins said, "inattention" by defense planners left a number of "major limitations" that "counterbalance" the improvements, so that "in some very important respects, we find that either we are no better off than we were in 1980 or our position is worse."

Progress in improving the U.S. strategic capability, he wrote, "has been least where impairment is most pronounced." While the sea-based leg of the American deterrent has been strengthened with new atomic submarines and missiles, the report said, land- and air-based weapons have advanced more slowly since 1980.

Without a missile defense system, the U.S. ability to protect its population and production base from nuclear attack remains "nil," Collins said.

The European theater nuclear capability is "no better" than in past years and is "backsliding in some respects," Collins said. The U.S. is deploying 572 medium-range ballistic missiles in Europe, but they lack the payload and range of Soviet SS20 multiple-warhead missiles targeted at Europe, he said.

The Soviet military and merchant marine "vastly outnumbered" that of the United States in 1980, according to the study, and "the gap is growing."

The quantitative U.S. edge in eight of 10 categories, ranging from strategic nuclear forces to naval destroyers, slipped in the past four years, the study said. Moscow, meanwhile, retained its numerical superiority in 16 of 19 areas and kept a "large edge" in manpower.