A senior Pentagon official who in the past has downplayed the potential consequences of a Soviet nuclear war, saying many could survive it, retreated before a hostile Senate committee yesterday and testified that neither superpower could win such a conflict.

T.K. Jones, deputy undersecretary of defense, joined assistant secretary of defense Richard N. Perle in an effort to persuade skeptical senators that the Reagan administration shared their abhorrence of nuclear war and did not consider one winnable.

In the course of a testy hearing lasting more than three hours, however, Jones was pushed into reiterating statements he has made in the past, including the contention that, because of Soviet civil defense preparations, 98 percent of the Soviet population could survive an all-out attack by American nuclear forces. Other experts have calculated that the Soviets would lose 50 million to 100 million people in such an attack.

Perle testified that the administration's request for $4.2 billion over the next seven years to expand civil defense programs in this country did not mean it thought the consequences of all-out nuclear war could be significantly mitigated.

Perle said, "We do not seek, nor do we believe that it is possible to obtain, levels of protection from the effects of all-out nuclear war that would reduce significantly the unspeakable horror of such an event."

Perle's testimony--apparently a response to charges that the administration is insensitive--appeared to contradict statements made by other officials as recently as Monday. Then, James Holton of the Federal Emergency Management Agency said the $4.2 billion Reagan civil defense program could double the number of American lives saved in a nuclear war.

In an earlier statement FEMA said: "The United States could survive nuclear attack and go on to recovery within a relatively few years."

But Perle testified yesterday that "the modest measures we are proposing for civil defense represent little more than insurance--insurance that in circumstances short of a central strategic exchange that is, all-out war , some lives might be saved that would otherwise be lost." He implied that in an all-out exchange the civil defense program would be insignificant.

Perle told the Senate arms control subcommittee that "Dr. Jones does not speak for the Defense Department on civil defense matters."

Deputy undersecretary Jones has made numerous public appearances and given interviews on civil defense. Some of his statements outraged the subcommittee's senators, who tried for weeks to get him to testify. Defense Department officials blocked Jones from testifying until yesterday.

Among the statements Jones was questioned about yesterday was a comment to the Los Angeles Times last January that "everybody's going to make it" in a nuclear war "if there are enough shovels to go around."

The shovels would be used to dig simple shelters, Jones said. "Dig a hole," he recommended, "cover it with a couple of doors and then throw three feet of dirt on top. It's the dirt that does it."

Jones did not challenge the accuracy of these quotations, which Robert Scheer of the Los Angeles Times has said were uttered during a tape-recorded interview. "The shovel matter refers to the kind of tools that are used to build Soviet civil defense shelters," Jones testified yesterday.

Jones did assert repeatedly that Scheer misquoted him as saying the United States--if it copies Soviet civil defense methods--could survive a nuclear war and return to pre-war levels of gross national product in two to four years. Jones said he had made this prediction about the Soviet Union, but never about the United States.

Last night Scheer, from a transcript, quoted him as saying: "If we used the Russian methods for protecting both the people and the industrial means of production, recovery time could be two to four years."