West German Chancellor Helmut Schmidt said today that the Soviet Union has dropped preconditions that blocked negotiations with the United States on limiting nuclear weapons in Europe and now is willing to begin new talks.

Schmidt, reporting to the West German parliament on his talks in Moscow earlier this week with Soviet President Leonid Brezhev, said the Soviets have dropped their demand that the West rescind its decision to station medium-range nuclear missiles in Europe.

He said the Soviets also are willing to start the talks before ratification of the second strategic arms limitation treaty (SALT II), now stalled in the U.S. Senate, but that the Soviets still insist that approval of SALT II would be necessary for any final agreement on Europe-based nuclear weapons.

The Soviets further suggested, according to Schmidt, that the prospective talks be broadened to cover not just the new missiles but America's so-called "forward-based systems" aleady deployed in Europe.

While Schmidt's account of his two days of talks with the Kremlin leadership left many questions about the Soviet position unclear, he cautiously called their offer a "constructive proposal" that creates a "new situation." But he said he would not call it a breakthrough in the difficult negotiations.

The Soviet proposal on nuclear arms talks is the main achievement of Schmidt's discussions in Moscow Monday and Tuesday with Brezhnev that also dealt, without apparent success, with the crises in Afghanistan and Iran. The only other substantive gain by Schmidt was the signing of a 25-year West German-Soviet economic cooperation agreement. Schmidt had withheld until today the details of the Soviet disarmament offer so that allied governments in Paris and Washington could be informed privately first. But as details surfaced, it was clear the West was left with a number of questions about just what the Soviets have in mind.

One of the key problems for Western military analyst is figuring out which weapons, in addition to the new missiles, to talk about. According to Schmidt, the Soviets named American "forward-based systems" a very loose term invented by the Soviets and not used by Washington.

Schmidt defined the term today as encompassing "U.S. nuclear weapons already stationed in Europe capable of reaching the Soviet Union." But it was unclear whether this meant sea-based as well as land-based weapons or what was understood s including Europe.

Schmidt sought nonetheless to put a positive, if cautious, cast on the expression of Soviet willingness to negotiate. "To me it seems important that this has created a new situation," he said, referring to the dropping of Soviet insistence that NATO reverse its decision to base nuclear missiles in Europe.

The NATO governments voted last December to produce and deploy 572 U.S. Pershing II and cruise missiles in Western Europe capable of reaching the Soviet Union, while at the same time extending an offer to the Warsaw Pact to begin negotiations on limiting the number of these weapons.

"I still do not see this as a breakthrough, since the success of such talks cannot be predicted," Schmidt said. However, I see a chance to prevent an unlimited arms race in this area." He said he had been impressed by the "great seriousness" of the Soviets about the prospective talks.

Schmidt said the Soviet proposal would have to be given careful study by the Western alliance in a "constructive spirit."

While aides to the chancellor have stressed that further progress on nuclear disarmament is chiefly a matter for the superpowers, it was evident that Schmidt felt he had inserted himself as a catalyst. As a result of his Moscow trip, Schmidt has emerged as a major spokesman for the West -- in this case, appearing even to have stolen an initiative from the United States, which initially had opposed his trip.

The Carter administration has strongly expressed concern that separate talks by Schmidt in Moscow could jeopardize NATO plans for stationing the new missiles in Europe and lessen pressure on the Soviets to withdraw from Afghanistan.

President Carter sent a strongly worded letter to Schmidt two weeks ago warning against any attempt to modify the NATO missile decision. Schmidt was angered by the letter but the two then met in Venice and said they had smoothed over any bad feelings.

As for this controversial proposal for East and West not to deploy their missiles for three years while they negotiate, Schmidt said the Soviet leaders rejected this as "unjust." It would have meant a halt in current Soviet deployment of SS20 missiles aimed at Western Europe, while the West continued to prepare for its planned deployment in late 1983.

Schmidt aides say they see the new Soviet offer as strengthening the NATO position. With Soviet objections to it removed, West German officials believe Belgium and perhaps the Netherlands, both of which have been reluctant to endorse the new missiles, will be more likely now to back the NATO decision.

Schmidt had said his main purpose in going to Moscow was to encourage the resumption of talks between East and West to forestall a worsening of world crises.He said today that this purpose had been fulfilled.

"It was proven that direct talk in a world marked by several difficult crises is not only necessary but useful," Schmidt said. At the same time, he noted that the international situation and especially East-West relations "remain difficult."

On Afghanistan, the other key issue in the Moscow talks, Schmidt said there has been no narrowing of differences. The Soviets rejected withdrawal from Afghanistan.

However, Schmidt said he is convinced that the Soviets have accepted that "the Afghanistan problem has to be resolved politically."

With West German elections three months away, the fact that Schmidt was able to win at least the appearance of a Soviet softening on disarmament is bound to help his already strong reelection chances.

Opposition Christian Democrats, critical of his visit at the start for undercutting the Western stance, had labeled it a campaign stunt and put pressure on Schmidt to come home with concrete results.

Today Franz Josef Strauss, the opposition candidate for chancellor, dismissed the Soviet offer as "old hat" and attacked Schmidt for overreaching himself.

"The hat was too big for you and the boots were too large. No German chancellor can mediate between the superpowers," Strauss said in response to Schmidt's speech.