President Reagan's promise on Monday to keep abiding by previous arms control agreements with the Soviet Union could be in conflict with an order he issued just a few days before: to study a new scheme for deploying the MX missile.

The possibly contradictory presidential actions reflect the dilemma faced by an administration that now seeks to demonstrate an interest in arms control to uneasy populations at home and abroad yet also still wants to beef up American nuclear forces quickly in the face of what the president claims is a "definite margin" of Soviet superiority.

Administration officials privately acknowledge the potential for conflict between the arms control promise and the MX order, which was to study a basing system called "Dense Pack." Dense Pack, some officials say, would appear to violate provisions of SALT II, the 1979 strategic arms limitation treaty between Washington and Moscow.

"It seems to be a matter of interpretation," Defense Secretary Caspar W. Weinberger said of the issue in a recent telephone interview. "Certainly there are some who say that," he added, when asked if Dense Pack would violate SALT II's prohibition against starting construction of "additional fixed ICBM launchers," meaning launchers for land-based, intercontinental-range missiles.

Even if the president does decide on Dense Pack as the way to base the MX, for which two successive administrations have now had trouble finding a home, the arms control issue might not come to a head until 1984, when construction would probably get under way for new underground silos to hold the MX.

The missile itself is scheduled to begin coming off the production lines in 1986. And it is conceivable that the new START or strategic arms reduction talks proposed by Reagan which open in Geneva June 29 might result in an agreement before the mid-1980s that would solve the problem.

On the other hand, Congress has insisted that that the administration decide by Dec. 1 on a permanent basing scheme for MX. And an announcement later this year that Dense Pack is the choice could touch off a major flap with Moscow and critics here and in Western Europe who might question the contrast with the president's Memorial Day remarks.

The 1979 SALT II agreement, although signed by Presidents Carter and Brezhnev, was never ratified by the United States. And President Reagan has now come out against ratification, arguing that SALT II would allow Moscow a big edge in the most threatening land-based missiles.

However, in an effort to placate some critics who argue that SALT II should be saved, and to maintain some of the restrictions in that agreement, Reagan said on Monday that the United States "will refrain from actions which undercut" previous arms agreements "so long as the Soviet Union shows equal restraint."

The problem, however, is that SALT II outlaws additional fixed-base ICBM launchers. Under SALT II, the Carter era plan for MX, which involved shuttling 200 MX missiles among 4,600 underground garages, would have been okay, according to letters of transmittal submitted to Congress by Carter. This understanding was never publicly accepted by the Soviets, but it is clear that Moscow understood the Carter administration's well-publicized MX intentions at the time.

Under the Carter plan, MX was billed as a mobile missile that carried its launching equipment around with it from one cement garage to the next in a shell game meant to confuse the Soviets. But Dense Pack involves a scheme in which new underground silos, probably just one for each missile and heavily fortified with concrete and steel as protection against atomic blast, would be built.

The idea is to bunch the missiles close together so that the blast from the first Soviet missile attacking one of the silos would, in effect, either destroy or blow off course following missiles aimed at other silos in the cluster.

Critics argue that this will force Moscow to build even bigger atomic warheads to try to knock our four or five silos at a time. They also argue that bunching MX silos so close together will allow the Soviets to pin down the entire force indefinitely and keep it from firing back because the Soviets could keep exploding bombs over the compact area. Administration officials say that about the only way to argue that Dense Pack is not a SALT violation is to claim that the metal container that holds the missile is actually the "launcher" and the silos are simply "holes in the ground." These officials acknowledge, however, that the Soviets undoubtedly "will scream blue murder" at such an explanation.

Others hint that the United States may, at some point, seek to defend Dense Pack partly on grounds that the Soviets also have violated SALT. The administration, thus far, has refrained from doing this but some officials believe some Soviet activities are suspect.

Another potential problem, according to government specialists, is that Dense Pack may not work without an anti-ballistic missile (ABM) system to protect it and this, too, could require breaking or amending the 1972 ABM treaty.

Weinberger calls Dense Pack "a very promising candidate . . . but every option we look at has some disadvantages and this is no exception." He has reconvened a committee of outside specialists, headed by Dr. Charles Townes, a University of California physicist who headed the same group last year to review MX basing options, to look at Dense Pack and some other alternatives that are still alive.

But "what we really need to do is, one way or another, get this missile MX in the ground" and deployed, he says. "That's the important thing," he believes, from the point of view of deterring the Soviets, matching Moscow's striking abilities, and trying to persuade Moscow to bargain earnestly at START.

Weinberger also makes clear that his preference on MX "at this time is to put it in the holes we've got," meaning existing Minuteman missile silos with improved protection as an interim measure until a permanent home can be found. Congress has ruled this out, however.

Asked if the United States would refrain from deploying MX as long as the START talks are continuing, Weinberger said "we would want to have a very firm agreement that made it clear that deploying this missile was no longer essential to maintaining a deterrent balance. But I think it's vital to have it," he said of MX, especially in case the talks break down.