The Soviet Union is going into a new round of nuclear talks with the United States confused about what the Americans are up to on what is seen as a "crucial" defense issue.

Looking ahead to the Geneva talks on reducing medium-range nuclear missiles in Europe, Soviet experts interviewed this week expressed persistent doubts about American intentions. Despite some relatively kind words about President Reagan that emerged late in the week, Soviet officials continue to see hostile mischief behind "completely unrealistic" U.S. proposals.

Moscow, they say, is serious in its intent to seek reductions in nuclear arms in Europe.

"The United States does not fully realize the crucial importance of the moment," one official said. "The outcome of the Geneva talks is going to shape our outlook for many years."

President Reagan's recent speech on arms control has left this capital divided when his offers of reductions are measured against what the Soviets see as an emerging U.S. nuclear strategy. As one Soviet source put it, Kremlin officials disagreed about whether the president's "gesture has any pragmatic value."

The Soviet view of U.S. nuclear strategy, outlined in background conversations by arms control specialists, is that the scheduled deployment of 572 Pershing II and cruise missiles in Western Europe would gravely shift the balance of power toward the United States.

The mobile Pershing would be "impossible to keep track of at any given time," the experts said. They said they also believe that the Americans plan to place cruise missiles in "exceptionally hardened silos which could be knocked out only by nuclear weapons."

The planned deployment is seen here as part of the hostility behind new American nuclear thinking. The Americans have come to believe, the experts said, that the winning side in a nuclear war would be one that would have its strategic reserves untouched.

If deployed, the Pershing II would change the strategic equation because it could hit Soviet command and control centers as well as strategic rockets located in the European part of the Soviet Union. Left untouched in any exchange, these experts say, would be the new U.S. sea-launched cruise missiles, viewed here as "the most important part" of U.S. nuclear strategy. Washington, they said, would then be in a position to "dictate" Soviet policy.

The deployment of the new missiles in Europe, they said, would require an "entirely new Soviet response."

The Soviets resent Reagan's "zero-option" proposal in which he asked them to dismantle their medium-range rockets in European Russia in exchange for nondeployment of the planned U.S. medium-range rockets. This idea is firmly rejected here -- but not in such vehement terms as when it was advanced.

Instead, experts here focus on the "generous" offer made by Soviet President Leonid Brezhnev in Bonn, stressing that "the spirit in which it was made is not less important than its content."

Brezhnev proposed a moratorium on all deployment of medium-range missiles in Europe during the Geneva talks. As a sweetener, he offered to reduce the number of Soviet systems in European Russia as "an advance payment" for the expected reductions in levels to be agreed upon in Geneva.

The types of systems and what is meant by "reduce" have not been disclosed. Experts here say that this would come up in the course of the talks. But, hinting that some systems may be dismantled, they urged questioners to study "the dynamics" of Soviet proposals on the issue.

They said there is room for maneuvering as far as "the technology of talks" is concerned, suggesting that Moscow may be willing to accept West German Chancellor Helmut Schmidt's idea that only land-based systems be discussed in the first phase of the talks.

The numbers and other details, they said, are not as important as political intentions and acceptance of the principle of "equal security" for both sides. According to the Soviet view, this means that U.S. forward-based systems must be included in the talks.

The experts also said that "one must get a deeper insight into British and French nuclear capabilities." When confronted with France's view that its force was not a part of NATO's structure and is intended to deter an attack on France, they said that "capabilities rather than intentions are important."

"If something should happen in Europe we have to count them on your side," one expert said. "This is pure logic."

The Soviets also express the belief that the Reagan administration lacks a clear nuclear policy and that its negotiators are coming to Geneva without a conception for the talks. But it was not clear here that the Soviets had any clear conception either.

The experts implied that both sides are going to lay everything on the table and that the process of sorting out what systems should be discussed and the definition of the area involved is bound to be long and complex.