Major proposal on conventional weapons; How Reagan would reduce East-West forces

President Reagan is preparing a major new proposal to reduce troops and conventional, or nonnuclear, weapons in Europe, according to administration officials.

Rounding out a series of arms control proposals, the President is expected to make public his plan for reducing conventional forces in Central Europe during his visit to Bonn next month in an appearance before the West German Bundestag, or parliament.

Talks on mutual reductions between the Western allies and Warsaw Pact nations have been held in Vienna over the past nine years, but they have failed to make much progress. One of the main problems has been what insiders call the data base, or data discrepancy, problem. The Soviet Union has claimed that the opposing forces in Central Europe are roughly equal. The West argues that the Soviets enjoy superiority in troop strength and have been failing to count about 150,000 troops on their side.

Some experts now profess to see signs that the Soviets are willing to be more forthcoming on the data problem. Two months ago, former Secretary of State Cyrus R. Vance said, after talks with Soviet officials, that he detected more flexibility on the part of the Soviets in this area. In an interview with the Voice of America on May 17, Eugene V. Rostow, director of the US Arms Control and Disarmament Agency, said, ''We are ready now in Vienna to make proposals. . . . That I think will show considerable promise for the first time of a breakthrough.''

Mr. Rostow declined further comment, but other officials said that the new Reagan proposal will suggest fusing into one comprehensive package earlier Western proposals for two phases in reductions. In the 1970s, NATO allies had proposed that the first phase result in the withdrawal of 68,000 Soviet troops and 29,000 US troops, as well as some equipment on both sides. The second phase would aim at having allies on both sides reduce their forces in order to reach collective ceilings that would be in rough parity.

The Reagan plan would also aim at equality of conventional forces, and would aim in particular at reducing the risks of surprise attack coming from the Soviet side. Many Western analysts have held that any Warsaw Pact attack on the West would probably take advantage of the Soviets' superiority in tanks and be based on a doctrine of mobility and surprise.

In his Nov. 18 speech last year dealing with middle-range nuclear weapons, Mr. Reagan touched on this problem. He declared that the defense needs of the Soviet Union ''hardly call for maintaining more combat divisions in East Germany today than were in the whole Allied invasion force that landed in Normandy on D-Day'' and said that he had already pointed out to Soviet President Leonid I. Brezhnev that to maintain peace, ''We must reduce the risks of surprise attack and the chance of war arising out of uncertainty or miscalculation.'' Reagan said that he was renewing a US proposal for an international conference that would reduce these dangers.

State Department officials see several advantages to the idea of fusing the previous proposal for two phases in reductions. To start with, the new proposal would have the advantage of simplicity, an official said, and thus ought to gain more understanding from the American and West European publics. This, in turn, the official said, might create more pressure on the Soviets to negotiate. And it might also help to focus public attention on the data problem, which, officials say, is still the main impediment to progress.

''The starting point for all this is the question of data,'' the official said. ''The Soviets, we think, got themselves into a box on this. . . . Perhaps we can recast, or redefine, the issue and help them get out of that box.''

President Reagan has already made two major arms control proposals, dealing first with medium-range nuclear arms and second with strategic nuclear weapons. By rounding his series of proposals out with new ideas on conventional forces in Europe, officials think that he will help to defuse the antinuclear movement in both the US and Western Europe. By making Bonn the launching pad for those new ideas, he might also help to defuse demonstrations directed against him during his tour of Western Europe.