President Reagan, long known for the stridency of his rhetorical broadsides against the Soviet Union, last night delicately balanced the tone of his nationally televised speech on the shooting down of Korean Air Lines Flight 007.

Before the speech, Reagan and his advisers concluded that the facts surrounding the Soviet missile attack in which 269 persons were killed were sufficient to stir outrage about "this crime against humanity," and that Reagan could make his point best by denouncing the attack but not taking a severely punitive approach to the Soviets.

So, the president struck notes of harshness in describing the "barbarism" of the incident but sounded a theme of restraint in cautioning against severe punishment.

He was stern in denouncing the specific episode, which he called the "Korean airline massacre." He said it was "the Soviet Union against the world and the moral precepts which guide human relations among people everywhere." He decried the attack as "an act of barbarism, born of a society which wantonly disregards individual rights and the value of human life . . . ."

But Reagan's rhetoric was tempered, too. He has been quoted as saying privately that there is no reason for "retaliation for retaliation's sake." Last night, Reagan ruled out action against the Soviets in the major forum of superpower talks--the arms race--and instead called for actions in the field of aviation safety that would prevent another such incident.

"It would be easy to think in terms of vengeance, but that is not a proper answer," the president said. "We want justice and action to see that this never happens again."

It was a "positive" response, in the words of presidential spokesman Larry Speakes, to an attack that Reagan called "murderous."

Originally, the White House had not considered a presidential speech to deal with the Soviet attack. The plan called for Reagan to inform the congressional leadership of his response, then have the White House announce it.

But Reagan has always had extraordinary faith in his own ability to shape U.S. public opinion with a speech. And last night's forum again gave him that opportunity.

It came after an event that seemed to reinforce boldly much of Reagan's view of the Soviet Union as an "evil empire."

His advisers realized soon after the incident that it had affirmed Reagan's deep convictions about the Soviet system.

But they and Reagan also realized that it was such a dramatic event that Reagan needed only to let the facts, detailed in tape recordings of the Soviet pilots' communications with their ground controllers, speak for themselves. Last night, the pilots were heard on national television.

While Reagan did not say "I told you so" about Soviet behavior, he coupled last night's condemnation of the attack on Flight 007 with an appeal for his defense buildup.

"There is something I've always believed in but which now seems more important than ever," Reagan said. "The Congress will be facing key national security issues when it returns from recess. There has been legitimate difference of opinion on this subject I know, but I urge the members of that distinguished body to ponder long and hard the Soviets' aggression as they consider the security and safety of our people . . . ."