After years of calling for the Soviet Union to accept deep cuts in strategic nuclear weapons, the Reagan administration suddenly had its wish fulfilled last Monday in Geneva. Now Washington policy-makers are trying to decide what to do about it.

The Soviet offer of a 50 percent reduction in strategic arms -- as Moscow defines them -- would bite much more deeply into the nuclear arsenals of the two sides than anyone here anticipated. If the cutback is really meant to be applied and is not just a new international public-relations gambit, it would require major changes in both Soviet and U.S. nuclear forces, according to U.S. experts.

Raising doubts here about the surprising offer are Soviet conditions and calculations attached to it that are unacceptable or unpalatable to the Reagan administration, especially the "cessation of work" on the Strategic Defense Initiative, the Soviet counting of U.S. Euromissiles and "forward-based" fighter-bombers as strategic weapons and a Soviet effort to ban new types of strategic weapons.

The central questions under discussion in the Reagan administration are whether and how to build on the positive aspects of the Soviet proposal while rejecting or negotiating changes in the negative ones.

No definite course of action has been agreed upon in the administration, sources said. For one thing, several key elements of the offer have not been defined by Soviet negotiators in Geneva, and some elements were mentioned for the first time by Soviet leader Mikhail Gorbachev in public statements during his visit to Paris, without any prior notice in Geneva. It may take weeks, or longer, to obtain full details of the Soviet offer, officials said.

Another problem of a more fundamental nature is the lack of agreement in what former undersecretary of state Lawrence S. Eagleburger called President Reagan's "coalition government" -- a coalition of those for whom ideology, especially anti-Soviet ideology, remains the highest value, together with those who accept the ideology but place the highest value on accomplishing concrete objectives. The latter group, but probably not the former, would like to see negotiations succeed and might be willing to compromise with the Soviets.

Until this week, Reagan held the unchallenged copyright on exciting but unnegotiable arms offers announced amid high drama. Reagan's "zero option" Euromissiles plan of November 1981, announced with fanfare in a televised address at the National Press Club, was later described by the secretary of state at the time, Alexander M. Haig Jr., as "not negotiable" and "absurd" as a serious offer, although attractive in public-relations terms.

And Reagan's strategic arms offer of May 1982, dramatically unveiled at his alma mater, Eureka College, was described by Haig as requiring "such drastic reductions in the Soviet inventory as to suggest they were unnegotiable."

The Gorbachev proposal, presented informally by Soviet Foreign Minister Eduard Shevardnadze at the White House Sept. 27 and put on the table in Geneva last Monday and Tuesday, was announced by Gorbachev amid fanfare Thursday before the French parliament in Paris. Earlier in the week, Gorbachev complained to French journalists about the Washington leaks of the main points of his plan, saying that Americans "always call on us to do everything in a confidential manner" but that "the whole world finds out within 10 minutes what happened at that confidential meeting."

The most attractive point of the Gorbachev offer, from the Washington viewpoint, is the part that grabs the headlines: the 50 percent cut, offered in terms of nuclear warheads, the currency U.S. officials have always favored. Such a proposal seemed almost unimaginable a few days ago and makes all others seem trifling.

One of the least attractive points in the offer is the weapons list from which this cut is to be made. It includes all U.S. intermediate-range missiles deployed in Western Europe as well as Air Force fighter-bombers at European and Asian bases and naval warplanes aboard carriers within range of Soviet territory. Soviet intermediate-range systems, including the SS20 missile and Backfire bombers, are excluded on grounds they cannot hit U.S. territory.

According to sources, the Soviets calculate that the United States has 3,360 "strategic nuclear delivery systems" and the Soviet Union has 2,504. The 50 percent cut over an unspecified number of years would apply to those figures.

The Soviets also calculate that a parallel cut in "nuclear charges" would produce an upper limit of 6,000 nuclear "charges" -- individual warheads, bombs and air-launched missiles -- on each side. Within this total, there would be a limit of 60 percent, or 3,600 charges, on any one branch of each nation's "triad," that is, land-based missiles, submarine-based missiles or bombers.

Ted Warner, Rand Corp.'s analyst of strategic systems and a former U.S. Air Force expert on the subject, said the proposed limit on land-based charges, especially, would produce "a surprisingly deep cut" in the strategic arsenals of both sides. If implemented, Warner said, these cuts would provide "a better chance to make a retaliatory force highly survivable" after an enemy attack, thus adding significantly to strategic stability.

"Neither of our countries has ever proposed as radical a solution as this" in negotiations, Warner said. He pointed out that Reagan's 1982 START proposal of 5,000 missile warheads on each side, no more than half of them land-based, also allowed each side 400 bombers that could carry several thousand bombs and air-launched cruise missiles.

Sen. Richard G. Lugar (R-Ind.), chairman of the Senate Foreign Relations Committee, and other lawmakers raised doubts about the Soviet limit last week, saying that the 3,600 land-based nuclear charges remaining after the proposed Soviet reduction uwould be enough to assign three warheads to each of the 1,025 U.S. land-based missile sites.

Warner, however, looked at the same figures another way. The Soviet Union now has about 6,400 land-based warheads aimed at the United States, he said, so the proposed reduction to 3,600 would be important.

Because of the need to balance its forces among land, sea and bomber-based elements, Warner added, the Soviets probably would have to reduce the total "throw-weight," the weight of the payload that missiles can deliver, of their strategic missiles from about 5.7 million kilograms to about 2.5 million to 3 million kilograms, about half.

The effect on U.S. forces of a 50 percent cut would also be great. If the United States were required to include its Euromissiles and "forward-based systems," as the Soviets proposed, only about a third of the allotted limit would remain for U.S. central strategic systems, according to official calculations. This is clearly unacceptable to U.S. officials. But even if the Soviet requirement is dropped during negotiations, U.S. forces would have to be restructured drastically to get under the 50 percent cutback.

The Pentagon probably would place about half its allowed 6,000 charges in its survivable submarine-launched systems, Warner said. The current land-based portion of the U.S. "triad" could not be protected against a first strike unless the Soviets are willing to go down to about 2,000 land-based charges, which "is not in the cards," he said.

But even at the proposed Soviet limit of 3,600, Warner said, "you'd have a much better prospect of making a mobile missile survivable on land." The United States is developing a small mobile missile called Midgetman.

John D. Steinbruner of the Brookings Institution said the proposed 3,600 limit on land-based charges "probably would not be good enough" to protect the U.S. land-based deterrent but suggested that a special category of "high quality weapons," including the Soviet SS18 and U.S. MX missile, be established with a low limit.