The Soviet Union is considering adopting a unilateral moratorium on deployment of its intermediate-range SS20 nuclear missiles in Europe now if a formula can be found for a joint Soviet-American declaration to push the Geneva talks on such weapons forward, a senior Soviet Communist Party official said yesterday.

If formally put forward by the Soviet leadership, such a proposal would represent a shift in the Soviet position, which until now has offered a moratorium only if the United States agreed in advance to cancel its planned deployment of 572 new intermediate-range missiles in Western Europe beginning in December 1983.

Stanislov Menshikov, the third-ranking staff official in the Soviet Central Committee's international department, said in an interview that Moscow "could start with some unilateral reductions in addition to the moratorium offer if we can assume that while we are talking both sides are not going to deploy missiles. We are prepared to begin this now and to move on to a second phase in which there would be agreements to substantial reductions in weapons to certain limits.

"A statement by both sides saying they were refraining from deploying anything in the area while these talks are in progress would create a good psychological atmosphere and enable us to go on to discuss concrete ways of reaching lower levels," Menshikov asserted. He said that while the Soviet Union would see this as a first step toward an agreement that would lead the United States to forgo deployment of the 572 Pershing II and cruise missiles, Washington would be able to go ahead with the December 1983 deployment if the Geneva talks had not made substantial progress by that point.

The Geneva talks on limiting nuclear weapons in Europe began on Nov. 30, with the U.S. delegation headed by Paul Nitze. Yuli A. Kvitsinsky heads the Soviet team.

Menshikov, in the United States to take part in a United Nations Association conference between Soviet and American citizens at Hershey, Pa., offered the view on the moratorium plan in explaining why the Soviet Union has rejected President Reagan's Nov. 18 "zero option" proposal. That proposal would entail the Soviet Union destroying its SS4, SS5, and SS20 missiles targeted on Europe in return for the United States not deploying the new generation of missiles sought by European leaders to counter the advantage that the SS20, in particular, gives the Soviet side.

It would leave NATO with a nuclear advantage in Europe since its "forward-based bombers" would not be affected, Menshikov asserted, although he acknowledged that American experts dispute the Soviet assessment of the balance of forces.

"That is something we can, and should discuss at Geneva," he said, adding that the Soviets are not insisting that the United States agree to include the forward-based systems at Geneva as a condition for the moratorium on new missile deployment in this early phase of the negotiations.

He said that despite the Reagan administration's continuing denunciations of Soviet actions, Moscow expects Washington to move forward on arms control in part because American and European public opinion demands such movement. Coming at a time of increased American and European concern about Poland, his remarks appeared designed to keep alive the Soviet effort to persuade foreign audiences of Moscow's reasonableness in nuclear arms talks in contrast to the trigger-happy image of the Reagan administration that Soviet propagandists have stressed.