The Soviet Union, in a shift that surprised the Reagan administration, substantially softened its demands this week for controls on the administration's Strategic Defense Initiative (SDI) program aimed at developing a ballistic missile defense, U.S. and Soviet officials said yesterday.

The officials said Soviet Foreign Minister Eduard Shevardnadze presented the new Soviet position during meetings here with Secretary of State George P. Shultz as one option in a package of new Soviet proposals intended to achieve a breakthrough on the contentious issue of SDI and to clear the way for a broader agreement on strategic, intercontinental missiles and other long-range weapons.

The new proposal would allow both sides to pursue research that was consistent with the Antiballistic Missile Treaty "as it was signed and ratified in 1972," the officials said. Previously, the Soviets have insisted on limits on SDI research that the administration has said are far tougher than those allowed under the ABM Treaty.

Some administration officials said the new proposal essentially overturns an intensive three-year Soviet campaign against any U.S. research under the SDI program and brings the Soviet position in line with that of the U.S. Senate, which voted Thursday to bar any SDI research outside the traditional, or "narrow," interpretation of the ABM Treaty.

At the same time, the Soviet position would still impose limits on SDI that the Reagan administration has shown no willingness to accept. Yesterday the Pentagon reaffirmed its support for SDI by announcing an acceleration of research and testing of six missile defense technologies to prepare for possible deployment in the mid-1990s.

The research and testing the Pentagon announced might be acceptable under the new Soviet proposal, but not deployment.

Shevardnadze suggested in a lengthy news conference at his embassy before his departure yesterday that the Soviet Union changed its position on the SDI program because it knows that it will not be abandoned by the Reagan administration.

Shevardnadze said the Soviets still felt SDI was "a bad program." But he added that "it is probably impossible to persuade the U.S. administration" on this point. "Therefore, we insist on a minimum solution, a minimum option," Shevardnadze said.

Senior U.S. officials have divided sharply on the proposal, with some expressing concern that the Soviets still want to hamstring the SDI program by blocking advanced research and development that could be pursued under the controversial "broad" interpretation of the ABM Treaty adopted by the Reagan administration in 1985 but repeatedly rejected by the Soviets.

Chief U.S. arms negotiator Max M. Kampelman said cautiously that "I think it is sufficiently different from other Soviet proposals that we have to study and pay attention to it, and we are going to do that." But he and other senior U.S. officials declined to predict the outcome of such a review.

President Reagan, asked at a White House news conference whether the United States would show some flexibility on SDI, passed the question to Shultz, who said, "We will never agree to restrictions that make it harder to pursue the Strategic Defense Initiative."

Reagan also complained that the congressional action restricting SDI tests was "foolish" and said it had "spoiled my day."

But Shevardnadze expressed optimism that the package of Soviet proposals would be seriously considered by the United States. "I believe this is not something in the remote future," Shevardnadze said of potential agreement on missile defenses and reductions in strategic weapons.

"I do hope that, after these talks in Washington, there will be some movement on the part of the United States," Shevardnadze said.

Viktor Karpov, chief of the arms control branch of the Soviet Foreign Ministry, also told The Washington Post that "now it is for the U.S. side to say yes or no." He added that the visiting Soviet delegation thought it was a positive sign that "we didn't hear any refusal" by the administration to discuss the new Soviet idea, as it had during previous meetings between senior U.S. and Soviet officials.

The visiting Soviet delegation of arms control and foreign ministry officials also proposed an alternative approach to SDI in which the two superpowers would "agree on a list of devices not to be put into space," including various missile defense weapons or prototypes of weapons. The two sides would also agree on "associated critical parameters" that would be used to describe such prohibited weapons.

"We have been proposing . . . to the United States for some time that our experts sit down together and decide what it means to strictly observe the ABM Treaty, what kind of work is permitted and what kind of work is prohibited," Shevardnadze said, referring to a series of proposals that the Soviets have introduced at the arms negotiations in Geneva.

On Sept. 11, for example, Soviet negotiators proposed several new detailed measures that could be used to differentiate between harmless space research and prohibited weapons testing or deployment. In a move that some U.S. officials have interpreted as an unusual display of Soviet flexibility, Shevardnadze proposed a series of even looser measures during discussions on Tuesday.

"We have been moving on this, but unfortunately it is not yet a two-way street," Shevardnadze told reporters.

A senior U.S. official, speaking on condition that he not be identified, said the administration position has been that no movement is needed on the U.S. side. "The ABM Treaty is a strong treaty, and we have not proposed a meeting" with the Soviets to discuss the detailed measures they have proposed, he said.

Paul H. Nitze, Shultz's senior arms control adviser, has long maintained that such discussions are needed to clarify ambiguities in the ABM Treaty about permitted or prohibited missile defense research. Some State Department analysts also maintain that the Soviets might accept limits on missile defense research that are so loose that they would not seriously impinge on SDI research plans.

But Nitze has been strenuously opposed by senior arms control advisers at the Pentagon and elsewhere, with the result that little progress has been made in discussions with the Soviets. Several U.S. officials said the new Soviet option, calling only for adherence to the "narrow" ABM Treaty interpretation for at least 10 years, was designed to overcome this obstacle by dropping the demand for detailed, negotiated definitions of permitted SDI research.

Shevardnadze emphasized that the Soviet Union is willing to put the U.S. and Soviet defense ministers at the heart of any detailed discussions on SDI, which could also include discussion of alleged Soviet violations of the ABM Treaty.

Shevardnadze said he had extended an invitation "to such a meeting" to Defense Secretary Caspar W. Weinberger during his meetings this week, but that Weinberger had not responded. A senior U.S. official confirmed the account, saying the reply had been, "Thank you very much. We'll get back to you."

One problem may be that the Soviets have proposed such a meeting be held under the auspices of the Standing Consultative Commission, a U.S-Soviet group that meets regularly to discuss treaty compliance issues. Just before Reagan's first summit meeting with Soviet leader Mikhail Gorbachev, Weinberger wrote Reagan that the group had never accomplished anything and should be disbanded.

The SDI programs approved by Weinberger for accelerated experimentation include a space-based missile interceptor; a ground-launched system that would attack warheads midway through their trajectory; space-based and ground-based surveillance and tracking systems and a battle management command and control system.

Defense Department officials billed the approval, which followed a vigorous internal Pentagon debate, as a major step toward the development of SDI programs. Announcement of the decision, originally scheduled for Tuesday morning when Shevardnadze arrived, was postponed until his meetings with Shultz ended.

Staff writer Molly Moore contributed to this report.