The Reagan administration may put an older Poseidon nuclear missile submarine in drydock this fall in an admittedly "gray area" effort to avoid direct violation of the SALT II strategic arms agreement when a new Trident submarine goes on sea trials, officials say.

With the unratified pact scheduled to expire at the end of the year, and with the Soviet Union exceeding the original limits for intercontinental missiles and bombers, officials say the administration intends to take a more flexible approach to compliance.

Taking a Poseidon out of active service -- but not dismantling its firing tubes, as would be required for strict compliance -- is one of several actions being considered by an interagency group to avoid violating the limit on multiwarhead ICBMs.

When the USS Alaska, a new Trident submarine with 24 intercontinental missiles aboard, begins sea trials later this year, the United States would be in violation of the SALT II limit of 1,200 multiwarhead missiles overall on land and at sea.

The Alaska's missiles would take the United States 14 ICBMs over the limit. A Poseidon carries 16 nuclear missiles. Under past practice, the United States would have 60 days after the Alaska sea trials begin to start the destruction of older multiwarhead missile systems.

This time, however, "we are looking at a departure from dismantlement in view of the widespread Soviet violations," a top Reagan official involved in arms control said. "We may just tie a Poseidon up, and wait."

Other steps being discussed, according to sources, include delaying the Alaska's sea trials, raising the question of whether such trials constitute deployment under the agreement, or delaying any compensatory move with the realization that the limitation expires at the end of this year.

"No agency is proposing strict compliance with the treaty as in the past," one arms control official said.

"We can interpret the treaty with greater flexibility," a Pentagon official said.

"There are lots of gradations of gray that can be applied."

Behind the current discussions, however, is a broader interagency debate pushed by Reagan officials who, in the words of one government arms control specialist, "want to get SALT II behind us." The immediate White House concern is drafting a June 1 report to Congress on the military impact of U.S. compliance to provisions of SALT II.

Although President Reagan has termed SALT "fatally flawed," his administration announced in 1981 it would not "undercut" the pact's limtation provisions as long as the Soviets also respected them.

One main target of compliance opponents is the Soviet approach to the limits. Under the pact, both sides were to reduce their total strategic missiles and bombers to 2,400 when the agreement went into force. Thereafter, they were to cut back to 2,250.

During the two years that Moscow awaited U.S. treaty ratification, Soviet missile and bomber numbers grew to more than 2,500. When the Reagan administration announced it would not seek ratification but would not undercut the treaty, the Soviets said they would not reduce to the treaty limits. Instead, they stated they would not go above 2,504, the number they had on that day.

Except for a disagreement over 16 bombers, U.S. intelligence analysts agree the Soviets have kept within the 2,504 ceiling. Nonetheless, critics argue the Soviets are 100 to 200 missiles or bombers over their allotment.

The president also has charged that the Soviets violated the treaty by building two new missiles rather than one and by encoding test data in violation of another provision of SALT II.

Last week, sources said the Soviets had told U.S. representatives in Geneva they are reducing the number of older silo-based SS11 ICBMs as they deploy new, mobile- or silo-based SS25s. Pentagon officials said the exchange "represented only a handful of weapons" and "was not significant."

The administration's treaty opponents joined conservatives in Congress several years ago to publicize what they said were Soviet treaty violations. The result was two presidential reports to Congress and a new emphasis in the Geneva negotiations on halting erosion of the 1972 Anti-Ballistic Missile Treaty.

More recently, a campaign was launched to point out a "double standard" in arms control compliance, an approach adopted first by the Pentagon and then last week by Reagan during his European trip.

Now, opponents inside the administration are arguing the United States would be in a better negotiating position with the Soviets if the SALT II missile and bomber limits were ignored.

Assistant Secretary of Defense Richard N. Perle reportedly has said that "continuation of SALT II levels even for a limited time would create a disincentive" for the Soviets to negotiate deep reductions in the Geneva arms control talks. Perle last week testified that the United States would be better off if the agreement were to lapse.

Perle has said the SALT II limits were approved by Moscow with the Soviet weapons-building plans in mind and that keeping the limits would not force the Kremlin to change those programs.

"I am not troubled that there is not an agreement around the corner," Perle told a Capitol Hill forum earlier this month. It is "worth waiting," he said, when the alternative is having to accept "agreements that permit them to keep their programs."

On the other hand, some administration officials and members of the Joint Chiefs of Staff say they think that there should be numerical limits on strategic systems while negotiations continue toward sharply reduced levels.

These "interim restraints," one official said, "are needed in the strategic field." Without them, he said, open-ended missile building would take place without any reductions. "That would cause political problems with European allies and require even deeper reductions just to get back to where we are now," he said.

Several recent studies by arms control groups and individuals who favor continuing today's limits have shown that without SALT II limits, the Soviets over the next five years would move even further ahead of the United States in missile launchers and numbers of warheads capable of knocking out U.S. missile silos.

Asked about that prospect, Perle said the Soviets have more than they need and that keeping their older missiles operational "doesn't matter."