One-fourth of the Soviet Union's non-nuclear military forces are committed to the Chinese border, Defense Secretary Harold Brown said yesterday in decrying "the simplistic comparisons" of U.S. and Soviet military strengths.

"The United States is the most powerful country in the world," Brown told the Commonwealth's Club of San Francisco. "The Soviet Union is not stronger militarily than the United States."

Brown's upbeat speech was, according to administration officials, designed to dispel public qualms about the U.S.-Soviet military balance.

These officials conceded that the tough talk out of the White House in recent weeks about Soviet and Cuban military activities in Africa set off some of the tremors.

In his speech Brown stressed that counting up the number of guns and tanks each superpower has to determine which one is stronger is a misleading way to measure relative strengths.

"We don't necessarily care whether the Soviets have more tanks than we do," Brown said. "We do care whether, in the event of a Soviet attack, we are able to throw it back. We assume - and I think with some merit - that if we can produce that kind of result, we will have produced a powerful deterrent to attack."

"And that," he continued, "rather than simply out-distanching the Sovies in number of tanks or any other single item is what you should demand of the U.S. and allied defense establishments."

He complained that "the simplistic comparisons" being broadcast these days ignore "that a quarter of the Soviet non-nuclear capability is on the Chinese frontier, that current Soviet operating doctrines require larger forces than we would use for the same purposes; and that we have stronger, more reliable allies than the Soviets."

Brown indicated that the Carter administration looks upon the threat China poses to Kremlin planners as a big plus for the West. One debate going on within the administration is how far the United States should go in supplying American weapons to China to draw Soviet forces away from the NATO front.

"A major two-front attack on our interests has become increasingly implausible with the continuation of the Sino-Soviet split" and the improvement U.S. relations with China, Brown said.

He said that therefore the "first and foremost" military concern of the United States "is the heavy concentration of Soviet general purpose forces in Eastern Europe and in the western districts of the U.S.S.R."

In discussing the Carter administration's nuclear strategy, Brown repeated that the heart of it remains mutual deterence - meaning that the United States will continue to have so many missiles pointed at the Soviet Union that the Kremlin could not hope to destroy all of them in a surprise attack and thus would be discouraged from launching a first strike.

In what the Kremlin might regard as a veiled threat, Brown indicated that the United States might adopt a "launch on warning" strategy for its land missiles if the Soviet Union keeps building nuclear block-busters designed to destroy them. Said Brown:

"The Soviets would have to consider the possibility that our minute-man missiles would no longer be in the silos when their ICBMs arrived. We have not adopted a doctrine of launch under attack, but they surely would have to take such a possibility into consideration."