Soviet officials said yesterday that they are prepared to call the third summit between President Reagan and Soviet leader Mikhail Gorbachev a success even in the absence of a new agreement on long-range strategic weapons.

In an apparent effort to lower expectations for specific accomplishments, one key Soviet official said in an interview that "with the improvement in atmosphere represented at the talks, our work here is mostly completed. We have a lot of work to do, but we are prepared to move forward."

When the summit opened on Tuesday, Kremlin officials said they were seeking an agreement to cut strategic weapons by 50 percent and a commitment from the Reagan administration to adhere to the 1972 Antiballistic Missile treaty for 10 years.

After two days of talks by the two leaders and their aides, the strategic arms negotiations remained inconclusive and were continuing late last night.

Soviet officials nevertheless said that generalized commitments by the Reagan administration to accelerate negotiations for strategic weapons cuts and to respect the Kremlin's effort to end its military involvement in Afghanistan were enough to call the summit successful.

"So far, so good," said Valentin Falin, director of the official Novosti information service, in an interview. "The main point is not to make new proposals but to find common ground for positive changes in the future."

The mood of superpower euphoria and the call for closer U.S.-Soviet relations sounded here by Gorbachev and Reagan and the generalized commitments to pursue arms cuts and withdrawal from Afghanistan justified declaring the summit a success, the officials said at the end of the second day of intensive talks.

This positive assessment marked a turn in the Soviet evaluation of U.S.-Soviet ties. Before the summit started, progress in relations between Washington and Moscow was stalled because the Reagan administration failed to meet the Soviet demand for arms control concessions and for an improvement in atmosphere.

Now Soviet expectations for the relationship are "more realistic," two Soviet commentators said in interviews.

"The main thing is to move forward step by step, not in giant leaps," said Vitaly Korotich, editor of the official magazine Ogonyok. "If it is possible to change something," he added, "we will do it. But we do not harbor illusions about the possibilities."

Fyodor Burlatsky, also an official Soviet journalist, echoed this view. "What we have now is detente without illusions," said Burlatsky, an editor for the official weekly Literaturnaya Gazeta. "Certain things are possible, and we will work for those."

"It is already certain the whole thing will be successful," Korotich said.

The lowering of expectations at the summit reflects a general shift in the Gorbachev reform movement away from grandiose, sweeping goals and toward more realistic objectives, Korotich said.

The shift, occurring in the past few months, is the result of a close Kremlin analysis of the results of the first 2 1/2 years of efforts made by Gorbachev to bring about major changes in foreign and domestic policy, other Soviet officials said.

As a result of the first two summits between Reagan and Gorbachev, the Kremlin concluded that the possibilities for major change in U.S.-Soviet relations are limited, the officials added. Both the November 1985 summit in Geneva and the October 1986 summit in Reykjavik, Iceland, ended without major conclusions.

"Even if there are no major strategic agreements in Washington," Vitaly Zhurkin, a deputy director of the U.S.A. and Canada Institute, said in an interview, "there will be in the future. I am certain of that."

Even the contentious issue of Reagan's Strategic Defense Initiative has receded, some Soviet officials said.

Moscow still considers an agreement from Reagan to adhere to the 1972 ABM treaty a prerequisite to an agreement for strategic weapons cuts on both sides, but Soviet officials said yesterday that, based on arms control talks during the summit, they believe that further agreements with the Reagan administration are possible.

Soviet officials accompanying Gorbachev stressed that the underlying success of the summit was not in the discussions of arms control or regional issues, however, but in the prospect for closer U.S.-Soviet cooperation established in the public appearances by the two leaders.

Throughout the second day of talks, Soviet officials were effusive in their praise of the new mood in relations and hopeful that it marked the first of many steps toward closer U.S.-Soviet ties.