The Defense Department revealed yesterday that the West has acquired highly sensitive information, apparently from Soviet sources, explaining in unprecedented detail the Soviet system for obtaining Western military and technological secrets.

This information was contained in a new Pentagon report released by Defense Secretary Caspar W. Weinberger, who told a news conference that the leakage of Western technology to the Soviet Union is "a far more serious problem than we had previously realized."

Among the Western technological secrets acquired by the Soviets, according to yesterday's report, were the fire-control radar for the F18 jet fighter, one of America's most advanced, and numerous computers and microchips, many of them used by the Soviets to make their own versions of Western electronic devices.

The report described a "Military Industrial Commission" in Moscow that coordinates efforts to beg, buy or steal technology by targeting American universities and U.S. defense contractors and hiring Western businessmen to assist in smuggling operations.

The report, which Weinberger termed "deeply sobering," adds another foreboding characterization of Soviet behavior by the Reagan administration as arms control talks resume today in Geneva and the president prepares for his meeting Nov. 19-20 with Soviet leader Mikhail Gorbachev. Weinberger said the report showed the need for increased vigilance in the West to try to frustrate Soviet espionage.

According to an informed source in the American intelligence community, much of the information in the new Pentagon report came from a Soviet KGB agent recruited in the 1970s by French intelligence. According to the source, this agent was part of "Line X," a team of about 300 scientific specialists working for Soviet intelligence who have been sent abroad on technology-hunting missions.

The KGB agent provided unprecedented information and documents to the French for a number of years, the source said. He was transferred back to Moscow several years ago, resumed contact with the French but then disappeared and is presumed dead, the source added.

In late March the Paris newspaper Le Monde published the first account of the documents on Soviet technological espionage, which is described in more detail in yesterday's Pentagon report. Le Monde said that information in the documents led to the French government's decision to expel 47 Soviet diplomats in 1983.

Compiled by the Defense Department, the Central Intelligence Agency and 20 other U.S. agencies, yesterday's report describes a Soviet apparatus headed by the Military Industrial Commission, consisting of executives of top military industries who select specific items for "collection," designate intelligence agencies for each job and earmark funds for each acquisition, a total of 500 million rubles per year in the late 1970s, the report said.

Yesterday's report translated that 500 million rubles into $1.4 billion in "1980 purchase power equivalents," but other specialists challenged that conversion. The Soviets' official exchange rate pegs the ruble at about $1.30, but on free currency markets it sells for about 25 cents. According to an economist who works on such issues for the CIA, the figure in the Pentagon report "really overstates the value of the ruble."

The information acquired by Western intelligence suggests that Soviet bureaucrats brag formally about how much money and research time is saved thanks to technology acquired from the West. The new report cites Soviet estimates of these savings -- for example, nearly 500 million rubles-worth of research saved between 1976 and 1980 just by two ministries, defense and aviation.

Soviet-bloc intelligence operatives managed every year in the late 1970s and early 1980s to obtain up to 10,000 pieces of military hardware and 100,000 engineering and research documents, the Soviets bragged, the report said.

Ninety percent of the documents acquired were unclassified, according to the report, providing the Soviets with, among other things, National Aeronautics and Space Administration studies of airframe design and flight computer systems for the space shuttle.

Altogether, the Defense Department report said, the Soviets have been able to improve thousands of weapons systems and research projects using Western technology.

Asked why the Pentagon released the report just days before Soviet Foreign Minister Eduard Shevardnadze is to visit Washington to prepare for the November summit, Assistant Defense Secretary Richard N. Perle replied: "Nothing is to be gained either in coping with this very serious problem or . . . in the larger dimension of Soviet-American relations by an artificially imposed silence on our own concerns."