National security affairs adviser William P. Clark, in a rare public appearance, yesterday accused the Soviets of "mass murder" and a "twisted mentality" for shooting down a Korean airliner and said the act showed the validity of President Reagan's long-held view of Soviet immorality.

"The sickening display of Soviet barbarism in the Korean Air Lines massacre shocked all of us," Clark said. "But at the same time, this dramatically brutal act must be deemed to be consistent with the behavior of a Soviet government that continues to terrorize and murder the Afghan people, using chemical weapons on Afghan villages; a Soviet government that sponsors the repression of the entire Polish nation."

Clark spoke to the Air Force Association convention here, where he accepted the Gen. H.H. (Hap) Arnold award in behalf of the president. Arnold, a pioneer aviator, is considered the architect of American air power in World War II.

Clark's speech followed the themes used by Reagan in a nationally televised address from the Oval Office on Sept. 5. But it was unusual for the harshness of its language, even by the standards of the Reagan speech, and for a suggestion that the Soviets would seize upon the incident to seek a summit meeting with the United States to reach a "greater understanding."

In fact, U.S. and Soviet diplomatic sources have indicated recently that they think the attack on the plane has reduced prospects for a summit.

"Time and time again, the Soviet Union has used the facade of negotiations while continuing their relentless military buildup," Clark said.

He nonetheless called for continued nuclear arms negotiations with the Soviets, which he said would be encouraged by congressional approval of the MX missile and the provision of "deterrence to continue to make clear to the Soviet Union that aggression would never pay."

Clark predicted that the Soviets would fabricate evidence to prove the "big lie" that "an innocent, stray plane was on a spy mission in the dark of night over Soviet Union islands."

He said that the Soviets, unlike the United States, "do use passenger aircraft for espionage purposes and have overflown the United States on spying missions."

"Neither our nor any government which holds life precious would consider mass murder as a response," he continued. " . . . We are disturbed but not surprised that the accusation of spying has been made--it is but further evidence of the twisted mentality of the Soviet regime."

Copies of the speech were distributed by the White House press office, calling attention to Clark's remarks as an expression of administration policy.