President Reagan's address to the U.N. General Assembly today relieved some of the tension that had been mounting steadily in the international arena for several weeks, and moved toward repositioning American diplomacy for the months ahead.

Reagan did not ignore the Soviet Union's downing of Korean Air Lines Flight 007, which has dominated world politics since Sept. 1, but he spoke in the world body of "the Korean airliner tragedy" rather than calling it a "massacre" or "crime," words he used as late as Sunday night here in a speech to Polish Americans.

For the most part, he spoke of this subject, and others, in personal and philosophical fashion, on the high ground above the passions of the moment rather than down in the trenches stirring them.

A top U.S. foreign policy official, in a briefing for reporters, said it is impossible to "put behind you" the airliner disaster in view of the "unanswered questions." But other officials said Reagan's speech reflected a recognition that the Korean airliner story has peaked, and that there is less profit than before in hammering away at it.

Reagan the U.N. statesman seemed to be saying between the lines that life must go on and the world must turn to constructive solutions to such dangerous issues as the buildup of nuclear weaponry and the eruption of warfare in Lebanon.

It was a speech tailor-made for the worried diplomats of the United Nations, the forum of the nonaligned and relatively poor and powerless nations. Ever since Reagan had agreed more than two weeks ago to appear here, U.S. Ambassador Jeane J. Kirkpatrick and others had hoped he would use the occasion to put the U.S. case in world affairs in a manner that the United Nations could accept, and that would get the 38th session of the General Assembly off to a good start.

Events contributed in major fashion to Reagan's ability to fulfill these hopes.

In the airliner case, the sheer passage of time had reduced the world's shock and anger to some extent. The moment for organizing sanctions or "political signals" against the Soviets has passed. A few hours before Reagan spoke, the Soviets took their first small step of international cooperation by turning over some of the wreckage of the downed airliner.

In Lebanon, the timing was even more propitious, due to the announcement of the cease-fire and of the beginning of a process of national reconciliation.

Nobody has withdrawn Reagan's accusation last week that "Soviet-sponsored aggression against Lebanon" is partly to blame for the bloodshed, but neither the president nor Secretary of State George P. Shultz has brought up the Soviet role since the truce was announced.

Instead there has been official praise for the "indispensable cooperation" of Syria, the Soviets' friend in the area, in reaching the cease-fire agreement.

The arms control passages that made up the central core of the address were enhanced by the new proposals made late last week to the Soviet Union in the Euromissiles negotiations in Geneva.

In this case the U.N. audience is secondary to the western European audience, the prize both Washington and Moscow are pursuing as the deadline for new U.S. missile deployment in Europe approaches.

Reagan's decision to announce the new proposals formally here added a note of reasonableness and willingness to compromise that contributed to the success of the address.

As the first passages of his speech proclaimed, Reagan in his U.N. incarnation was "preoccupied with peace" rather than nailing down the responsibility for its absence in many parts of the world.

To have stirred more Cold War conflict in the speech, one U.S. official said, might well have destroyed any chance that the current U.N. session could take constructive stands and, in at least a few cases, authorize constructive actions. One such possible action, which was the subject of inconclusive high-level discussions today, would be authorization of a U.N. role in supervising the Lebanese cease-fire.

Reagan's references to the requirements and importance of "genuine nonalignment," though possibly obscure to some of his domestic audience, were considered of major importance in the U.N. forum. They reflect growing U.S. unhappiness that "nonaligned" nations are often unwilling to take a stand against the Soviet Union and at times are amenable to sharp criticism of the United States.

Recently the United States and its allies were unable to line up enough nonaligned nations to back up a U.N. role in the Lebanese conflict or to take a stand in the struggle in Chad. The U.N. Security Council was able to muster only the barest of majorities for a stand on the Korean airliner downing because such nations as China and Zimbabwe would not agree.

While rejecting as "false and misleading" a view that the world is divided into empires of East and West, Reagan held out a welcome on this side for truly nonaligned and unsubservient nations and attacked the "pseudo-nonalignment" of Soviet-dominated governments that "have long since lost their independence." He managed to do this in a manner that did not anger or offend, while making his point completely clear.

It is too much to suggest that Reagan's speech announced a major new policy turn in any element of U.S. diplomacy. Nevertheless, it did seem to signal a lowering of voices and a raising of hopes in the world community.

For today at least, diplomacy was "the most honorable of professions" and Reagan an advocate of its triumph. The U.N. delegates here, and perhaps a wider audience beyond, exhaled noticeably on hearing this message after a period of high international tension.