A senior Soviet arms control specialist was quoted today as saying that the Soviet Union may adopt a "launch-on-warning" defense posture--which provides for quick firing of nuclear missiles--as a relatively cheap way of responding to President Reagan's planned buildup of U.S. strategic might.

Under "launch on warning," Soviet nuclear missiles capable of reaching the United States would be programmed for almost instant action if computerized Soviet intelligence monitoring facilities reported an imminent American threat to the Soviet Union.

A group of American academic specialists and antinuclear activists, who asked that the Soviet official not be identified, quoted him as saying that "launch on warning" would almost "eliminate the human element" from calculations in emergencies.

From the Soviets' point of view, the new posture would better protect their missiles from a surprise U.S. strike. But it would also increase the possibility that the Soviets would launch an attack by mistake because of computer error.

"We cannot afford to match you ruble for dollar," the official was quoted as saying by the Americans, "but you'd be making a great mistake to think that you could gain strategic superiority" over the Soviet Union.

The new posture would be extremely dangerous and destabilizing, foreign experts here said. The Soviet specialist was quoted as bemoaning that controls over nuclear weapons are "more rigid here and therefore such a change would be more dangerous."

The Americans met a number of top officials in Moscow and said the Soviets think that the United States is "pulling ahead in the arms race." As a result, the suggestion that Moscow may adopt "launch on warning" may have been calculated either to convey the gravity of Soviet concerns or perhaps to increase pyschological pressure on Washington by signaling that automatic massive retaliation is being seriously considered.

In the United States, computers gave false alarms in November 1979 and June 1980 that the Soviets had launched nuclear attacks.

The 10-member American delegation was sponsored by Washington's Institute for Policy Studies, a think tank associated with leftist policies in international politics. It included former U.S. representative Donald Fraser, now mayor of Minneapolis; the Right Rev. Paul Moore, Episcopal bishop of New York; Marcus Raskin, a senior fellow at the institute; Robert Borosage, institute director; Howard Frazier, executive director of the Foundation Promoting Enduring Peace; Roger Wilkins, senior fellow at the Joint Center for Policy Studies, and Robert Potter, an attorney.

During their five-day visit here, the Americans reportedly conferred with five members of the Soviet Central Committee including Vadim Zagladin and Georgi Arbatov, and more than a dozen senior Soviet arms control experts including Oleg Bykov.

The Americans--who said they raised the issues of Poland, Afghanistan, Jewish emigration and chemical weapons with their hosts--came away with the impression that the Soviets were primarily concerned with the growing likelihood that the arms race will center on counterforce weapons, or nuclear weapons capable of destroying an enemy's land-based nuclear missiles.

The Soviets believe that Reagan is trying to build up the U.S. ability to knock out their land-based missiles--which account for 70 percent of the Soviet strategic force--by developing the MX missile and the D5 warhead for the submarine-launched Trident missile and by planning to deploy Pershing II and cruise missiles in Western Europe, the Americans said.

The Soviet officials were quoted as saying that they were prepared to develop "countermeasures perhaps not as expensive" as the American programs.

The Americans also reported that Soviet officials said they were prepared to make "unilateral initiatives" in an effort to stop the arms race. The Soviets were quoted as saying that a proposal by Rep. Albert Gore Jr. (D-Tenn.) calling for deep cuts in nuclear arsenals could form the basis for future arms negotiations.

Regarding other issues, the Americans quoted one Soviet official as saying that a meeting between Reagan and Soviet President Leonid Brezhnev "would be useful" but that Reagan's plan to entertain prominent Soviet dissidents on May 5 suggested that "he cannot be serious" about easing tensions between the two countries.

One official made it clear that the Soviets were wary of Reagan's proposal for a summit meeting during a special U.N. session in June. He was quoted as saying that "Brezhnev probably will be there," but the Americans added that the remark appeared to reflect only the official's personal view.

The 75-year-old Soviet leader, who was hospitalized on March 25 after his health deteriorated on a visit to Soviet central Asia, was reported to be convalescing at his country home outside Moscow. The Americans reported that none of the Soviet officials made any reference to Brezhnev's health.

The Institute for Policy Studies and the Soviet Institute on the U.S.A. and Canada, a government think tank, have signed a protocol establishing annual Soviet-American conferences to deal with the issues of bilateral relations and disarmament. The first meeting was set for next year in Minneapolis with 40 representatives from each side.