Skeptical Reagan administration officials said yesterday that an anticipated new Soviet proposal to reduce strategic nuclear weapons could be, as one arms control analyst put it, "either a small step forward or a large step backward" from earlier Moscow proposals.

"The devil is in the details," a key Pentagon official said yesterday, voicing what several officials said was the initial reaction to the "vague" descriptions of the offer hinted at by Soviet officials and now circulating within the U.S. government.

As described by Soviet sources over the past two weeks to members of Congress and other U.S. government officials, the proposal calls for reductions by both superpowers of nuclear delivery systems and warheads by 40 percent.

The proposal described by these Soviet sources would also limit the concentration of both superpowers' nuclear arsenals in any one of the three broad types of nuclear systems both maintain.

The Soviet proposal is described as requiring that no more than 60 percent of either country's nuclear arms be deployed in land- or submarine-based missiles, or on bombers.

As presented informally to U.S. officials, the Soviet package also calls for restraints in President Reagan's Strategic Defense Initiative, the "Star Wars" research program to invent a defense against nuclear missiles.

Soviet sources have said Foreign Minister Eduard Shevardnadze, who met yesterday with Secretary of State George P. Shultz in New York, will present the proposal to Reagan at the White House on Friday.

The seriousness of the Soviet proposal in the administration's view, according to one government arms control expert, will initially be judged by how the proposed reductions and limitations are calculated.

The United States has a large lead over the Soviets in bombs and cruise missiles, which the administration considers less threatening, whereas the Soviets have a comparable lead in the missile warheads. If the weapons were counted as equal, officials said that could set back attempts to find an arms control solution.

Despite the U.S. skepticism, the official said he expected the administration to encourage presentation of the proposal in detail at the recently resumed Geneva arms talks, where the issues could be examined and made the basis for negotiations.

The overall U.S. strategic force is about equally divided among nuclear bombs and warheads carried by bombers, submarine- and land-based missiles. The United States has 7,800 warheads on its strategic missiles, 5,800 of which are aboard submarines.

The overall Soviet strategic force is found primarily on its land-based missiles. Almost 70 percent of its explosive force is carried by 1,400 land-based intercontinental ballistic missiles (ICBMs), which have roughly 6,400 warheads. Roughly 1,000 sub-launched Soviet missiles carry another 2,800 warheads.

Of the Soviet land-based ICBMs, 308 are the giant, 10-warhead SS18s, which are much larger than any U.S. missile, including the MX now under development. In addition, the Soviets have 360 SS19s, a smaller ICBM but still bigger than any U.S. missile.

Reagan administration officials say these two ICBMs, and particularly the SS18s, threaten the American land-based missile force because their 4,500 warheads could knock out the 1,030 U.S. land-based missiles in a surprise first strike.

The Soviet bomber force is considered less threatening, though some Reagan officials say it is larger than the active U.S. B52 force.

The focus of U.S. arms control proposals under the Reagan administration has been to sharply reduce the presumed advantage the Soviets have in ICBMs stemming primarily from the SS18s.

The U.S. proposal, now on the table in the Geneva talks, would do this in two steps. It calls for each side to reduce its overall number of missile warheads to 5,000; it also would limit to 2,500 the number that could be placed on land-based missiles.

To get directly at the SS18 force, the U.S. plan initially called for a specific reduction from 308 to 110 missiles. In the most recent offer, the U.S. proposal suggested an overall limitation on the amount of throw-weight, or the volume of nuclear explosives, that could be carried on missiles.

In that category, the present Soviet missiles could launch 5.6 million kilograms; the U.S. plan wanted both sides to be limited to 2 million kilograms, or roughly the capacity of the much smaller U.S. ICBM force.

The last Soviet proposal, made during the 1981-83 Geneva START negotiations, called for a reduction on each sides' nuclear delivery systems from the SALT II level of 2,250, to 1,800. No specific limit was proposed for warheads but Soviet negotiators talked of "nuclear charges," a category they said should also include bombs and cruise missiles along with ICBM warheads.

If the 40 percent reduction, said to be present in the Soviet plan, is applied to the SALT II numbers, one official said, it would represent a cut to 1,300 launchers. And if 60 percent of that force could remain in land-based ICBMs, the Soviets would still have 780 ICBMs, including the 308 SS18s.

"That's not very promising," a Pentagon official said yesterday, while conceding that it was better than two years ago.