The downing of an Iranian plane by a U.S. Navy warship might in an earlier time have put great pressure on U.S.-Soviet relations, but the reaction of both superpowers has been unusually restrained, reflecting continued post-summit good feelings.

Moscow's criticism of the U.S. action has been circumspect while the Reagan administration is downplaying previously stated fears of Soviet expansionism as a rationale for the U.S. military presence in the gulf.

The Soviets have used the incident to renew their call for the withdrawal of the U.S. naval fleet from the gulf but have stopped short of the type of inflammatory rhetoric that the Reagan administration used to denounce the Soviet attack on a Korean passenger plane five years ago.

The toning down of rhetoric has been evident on the U.S. side as well.

Just a year ago, former Defense Secretary Caspar Weinberger had said that the United States must not retreat from the gulf because "we simply cannot allow the Kremlin to have its will over the region."

But now, perhaps with the afterglow of President Reagan's visit to Moscow still visible, the administration has been citing other factors to defend its gulf policy.

The State Department now says the the U.S. goal in the region is to bring the Iran-Iraq war to an end and to protect the security of U.S. friends in the area. References to the Soviet Union have been non-existent.

All but forgotten is the almost casual way the United States decided in early 1987 to expand its military presence in the gulf, when the administration decided to reflag Kuwaiti tankers after a number of these vessels had been bombed by Iranian planes. The new policy entitled the reflagged tankers to the protection of the U.S. Navy.

The administration took the step as a gesture of concern after the Soviets began helping out Kuwait by leasing to the Kuwaitis three Soviet tankers.

There were some objections at the time, but it was only after an Iraqi jet, in a case of mistaken identity, attacked the USS Stark in May 1987 that the wisdom of the administration's course was widely _ and sharply _ questioned.

The lives of 37 American sailors were lost, and there was concern once again that use of the U.S. military to make a symbolic statement in a Middle East trouble spot was a riskier policy than it had seemed initially.

In October 1983, use of U.S. forces in Lebanon in an attempt to promote peace led to the deaths of 241 Marines after a truck bombing at a military headquarters in Beirut.

State Department officials acknowledge that concern about Soviet domination of the gulf has received less emphasis lately from administration spokesmen.

The Kremlin, meanwhile, has been showing far more restraint following Sunday's incident than the United States did in September 1983 when the roles of the two countries were reversed.

The Reagan administration reacted indignantly when Soviet fighter planes shot down the Korean passenger plane, causing the deaths of 269 people. The administration called the incident "calculated, deliberate murder."

In Moscow on Monday, Soviet Foreign Ministry spokesman Gennadi Gerasimov soft-pedaled his criticism of the United States even though, he said, the downing of the Korean airliner was far more understandable than Sunday's attack on the Iranian plane.

Whereas the Iranian plane was flying in daylight over international waters, the Korean plane had intruded into Soviet airspace at night, Gerasimov said.

But the chairman of the joint chiefs of staff, Adm. William J. Crowe Jr., said Sunday the two incidents were not comparable. He noted that the Iranian plane was flying over a war zone where there was combat in progress and that, in contrast to Sunday's incident, the Korean plane _ unlike the Iranian airliner _ had received no warning before it was shot down.

The 1983 incident produced serious strains in Soviet-American relations, poisoning the atmosphere of a meeting held shortly thereafter between Secretary of State George P. Shultz and then-Soviet Foreign Minister Andrei Gromyko.

But Gerasimov has been a model of restraint since the Sunday incident. Alluding to the "wild anti-Soviet howl" of 1983, he said that Moscow has no intention of responding in kind with the shoe now on the other foot.

"I don't think we will follow that bad example," he said.