The Soviet Union proposed yesterday at the Geneva arms-control talks to abandon its eight-month effort to negotiate a treaty with the United States on space-based missile defenses.

On the second day of a new round of meetings, chief Soviet arms negotiator Alexei Obukhov proposed instead that the contentious space weapons issue be settled by a special protocol to any U.S.-Soviet treaty sharply reducing strategic, or long-range, offensive nuclear weapons.

The protocol would have the same legal status as the treaty.

Several U.S. officials said the draft Soviet protocol appeared to be consistent with the Soviet position at the close of the Washington summit meeting last month between President Reagan and Soviet leader Mikhail Gorbachev.

The two sides agreed in a joint communique then to observe the Antiballistic Missile (ABM) Treaty "as signed in 1972, while conducting their research, development and testing as required."

The Soviets said however this meant that realistic SDI space tests would be barred, while the Reagan administration said they were permitted.

The Reagan administration quickly rejected the Soviet proposal yesterday on grounds that a strategic arms treaty should not be tied to constraints on Reagan's Strategic Defense Initiative (SDI) space weapons research program.

Terry Shroeder, a spokesman for the U.S. negotiating team in Geneva, said the delegation had hoped "the Soviets {would} take a more realistic approach" in the discussions, which resumed Thursday after a month's recess.

He described the Soviet proposal as a "renewed attempt to achieve positions that we previously rejected."

The Reagan administration decided last month after a long internal debate to begin drafting a U.S. version of the draft space weapons treaty introduced by the Soviets last May.

The U.S. objective is to obtain assurances from the Soviet Union that SDI space tests are not constrained by the 1972 treaty.

Several U.S. officials said the Soviet proposal demonstrated not only that the two sides remain at odds over the SDI program, but also that they are headed in opposite directions in the negotiations at the outset of their ninth round.

Speaking to reporters after an hour and 40-minute presentation to U.S. negotiators, Obukhov said, "It would be particularly important to make legally binding. . . compliance and nonwithdrawal from the ABM Treaty for an agreed period."

U.S. officials said they were uncertain whether this Soviet formula would replace, or merely complement, a previous Soviet demand that a treaty reducing strategic arms would "cease to be in force if either party proceeded with practical development and deployment of an ABM system beyond the provisions of the ABM Treaty."

Some U.S. officials, including Shroeder, suggested the Soviet proposal violated an agreement between Gorbachev and Reagan to settle the space weapons issue in a separate treaty.

But others said the two sides had merely decided to resolve the issue in an agreement with "the same legal status" as the strategic weapons treaty or the ABM Treaty.

"We tried to get the Soviets to agree to having a separate treaty, but they refused," said a knowledgeable arms-control official.

Several officials said the Soviet proposal reflected Gorbachev's willingness to forgo arduous and time-consuming negotiations with the Reagan administration on a detailed space weapons agreement, and his interest in pressing ahead with a strategic arms accord.

But they said such an accord may not be possible unless Gorbachev abandons his insistence that it include a ban on realistic SDI tests.

On another arms-control topic, a U.S. delegation completed an unprecedented five-day visit to the Soviet nuclear test site at Semipalatinsk in south-central Russia yesterday.

Robert B. Barker, an assistant to the secretary of defense for atomic energy and leader of the 20-member U.S. group, said the Soviets had "fully complied" with an agreement last fall to provide information on nuclear testing procedures and access to the test site in preparation for measuring the force of a Soviet nuclear blast later this year.

"It's a very good model for future arms-control measures when two nations can have technical experts sit down together and decide" on new measures to verify compliance with two existing treaties limiting the force of nuclear tests, Barker said.

A team of Soviet inspectors is to visit the U.S. nuclear test site in Nevada Jan. 25-30.