Secretary of State George P. Shultz said yesterday that the decision of Ukrainian sailor Miroslav Medvid to return to the Soviet Union may have been based on fear of reprisals against his parents.

As Medvid's ship steamed through the ocean toward home, Shultz defended the administration's handling of the ship-jumping incident.

Shultz said that every U.S. agency involved, including the Justice Department, had concluded that, for whatever reason, Medvid had changed his mind about defecting.

The incident created problems for U.S.-Soviet relations shortly before the summit between President Reagan and Soviet leader Mikhail Gorbachev in Geneva Nov. 19 and 20, and drew sharp protests from the Republican Party's right wing, led by Sen. Jesse Helms (R-N.C.).

The 120,000-ton grain-carrying freighter Marshal Konev was allowed to leave Mississippi River waters in Louisiana Saturday and sailed into international waters late that night, despite a subpoena from the Senate Agriculture Committee.

Protest boats were watched carefully by the Coast Guard to prevent any incidents.

"It would look as though he Medvid decided, somehow, that he wanted to come to the United States and after he was subjected to whatever he was subjected to aboard that ship, he changed his mind," Shultz said on CBS' "Face the Nation."

The secretary called attention to the report of a U.S. Air Force psychiatrist who interviewed Medvid after the sailor first was returned to the ship by the border patrol and then removed by U.S. authorities for further questioning.

Shultz noted that Medvid "repeatedly referred to Mama and Papa, and he talked about life."

"I have no basis for being explicit about what threats may have been suggested to him," Shultz said, "But it is clear that he had his mother and father on his mind and, all things considered, decided what he wanted to do."

Shultz conceded, as he had done previously, that it was a mistake to return Medvid to the ship against his will after he had twice jumped into the Mississippi.

But Shultz asserted that everything possible subseqently was done to correct that mistake.

He noted that Medvid had been taken off the vessel, was allowed to rest, was given physical and psychological examinations and was questioned anew "in a nonthreatening environment" about whether he wished to remain in this country.

The 25-year-old Medvid was returned to his vessel by Border Patrol agents after jumping ship at Belle Chasse, La., on Oct. 24.

Four days later, Medvid was removed from the ship for lengthy interviews with State Department officials, and he eventually signed a statement declining political asylum.

Helms, chairman of the Agriculture Committee, issued a last-minute subpoena for Medvid.

But Shultz said the final decision on allowing the Marshal Konev to depart was up to the Justice Department, and that "it was the consensus of all the people working on this . . . representatives of the Department of Transportation, Justice, State and so on . . . that there was no legal basis to hold the ship."

Shultz also was questioned about Vitaly Yurchenko, the KGB official who allegedly defected to the United States last August only to turn up last week at the Soviet Embassy here claiming that he had been kidnaped, drugged and brought to this country against his will.

"He is a high person in the Soviet KGB," Shultz said in response to questions about whether Yurchenko might have been part of a plot to embarrass the United States on the eve of the summit.

"My opinion is that he defected and, for some reason, changed his mind," the secretary added.

Shultz refused to discuss what the United States had learned from Yurchenko during the three months that he was in U.S. custody.

"But he was not given any U.S. secrets at all," Shultz said.

Shultz, discussing the summit, described as not quite accurate reports that Reagan's national security affairs adviser, Robert C. McFarlane, had said there would be no joint U.S.-Soviet communique.

"What the national security affairs adviser said was that it was unlikely," Shultz said. "We have to report whatever happens at the big Geneva meeting. What form that takes remains to be seen. It depends on what amount of things are put together at that time."

Shultz also implied that the United States might be establishing a link between progress in the U.S.-Soviet negotiations in Geneva on reducing nuclear missiles and continued U.S. adherence to the SALT II treaty.

The treaty has not been ratifed, but the administration has adhered to the treaty's restraints on the production and deployment of intercontinental ballistic missiles.

Asked whether Reagan will continue this voluntary compliance, Shultz replied: "What he has said is that adherence to the SALT II treaty depends on Soviet behavior, and ability to have our negotiations go forward toward lowering offensive arms levels. And that's the test."

The Reagan administration has said frequently that the continued adherence of this country would depend upon Soviet behavior in complying with the SALT II provisions.

But the secretary of state's comments concerning negotiations with the Soviets on offensive weapons appeared to establish a link to the negotiations in Geneva that had not been stated before by the United States.