Despite the largest U.S. defense-spending spree in peacetime history, the Pentagon has achieved "minuscule improvements" in the nation's military readiness, technology and weapons inventory, Rep. Les Aspin (D-Wis.) charged yesterday.

Aspin, chairman of the House Armed Services Committee, said an analysis of the $1 trillion spent on defense the past four years raises the question: "Is Ronald Reagan doing with defense what he accused previous administrations of doing with social welfare -- just throwing billions at the problem?"

Aspin asked the question as his committee prepared for hearings billed as a report card on Reagan's military-modernization program. The hearings, which are to begin today, reflect growing congressional scrutiny of defense spending.

The Defense Department, in a rebuttal, said readiness of key weapons systems has "improved markedly," aircraft and sealift capabilities have been "dramatically improved," military training has expanded and the cost of many weapons has declined. The rebuttal cited a halving in the price of AIM9 air-to-air missiles and a 25 percent cut in the price of Bradley fighting vehicles.

"Progress has been made," the Pentagon statement said. "Our military strength has improved over the past four years, but there is much to do."

In his report released yesterday, Aspin cited "dramatic improvements" in personnel as the only unqualified success in the Reagan years. While manpower expenses rose a modest 10 percent from 1980 to 1984, he said, the military attracted a larger number of high school graduates and increased its reenlistment rate.

But his assessment of other key measures of military capability, Aspin said, shows that improvements are "at best, marginal." The report contends that:

*Despite a 56 percent rise in funds for defense research and development, U.S. military technology progressed little compared with that of the Soviet Union. Of 15 technologies ranked in 1980 and 1985, the U.S. position improved only in radar sensors while registering no change in 11 technologies and losing ground in three -- submarine detection, electro-optical sensors and microelectronic materials.

*Despite spending increases of 81 percent for centrally managed material readiness, there was no commensurate improvement in such categories as equipment maintenance and Air Force and Navy flying hours.

*Despite a 91 percent spurt in weapons budgets, the inventory of a third of military equipment categories -- including sealift ships, heavy mortars, antisubmarine warfare aircraft and strategic nuclear launchers -- shrunk in size.

Part of the explanation for declining inventories in the age of lavish defense spending, Aspin said, is the rise in unit price for certain weapons. For example, the cost of fixed-wing aircraft has increased from $20 million per plane in the 1970s to $27 million in the past three years, the report said.

As prices rose, the report continued, the number of fixed-wing aircraft purchased has declined, from 2,002 during the Carter administration to 1,757 in the Reagan years.

Aspin said the "soaring" unit costs raise "grave questions" about Pentagon management. "Are Pentagon managers sloppy because they are operating in a resource-rich environment?" he asked. "Are we paying more for weapons because contractors know a surfeit of money is available?"