It would be our basic policy" to retaliate strongly if the Soviets or their surrogates in Syria launched attacks in Lebanon, Defense Secretary Caspar W. Weinberger said here today.

He told the American Jewish Committee that there is an "ever-increasing amount of Soviet strength going into Syria," including troop units, "not just as advisers or trainers."

"This stepped-up Soviet involvement in the region makes a solution to the crisis in Lebanon more difficult to achieve," Weinberger continued, "and heightens the danger of direct conflict between Syria and Israel."

Asked what the Reagan administration would do if the Soviet Union turned Syria into another Cuba in the sense of using it as a launching pad for military expeditions, Weinberger replied, "The simplest way to deal with a problem of that kind is to make it very clear to the Soviets or to any proxies that they may have in Syria that any aggression by them would be met by a retaliatory force."

The response would be so strong, Weinberger continued, that it would wipe out "any kind of hope of gain to the aggressors."

Weinberger said that this "certainly would be our basic policy" if the Soviet Union or its surrogates use Syria as a base for military adventurism.

What would sharply reduce tensions in the Middle East, Weinberger said, would be a withdrawal of Israeli, Palestine Liberation Organization and Syrian troops from Lebanon.

"I think we're reasonably close to such a solution," Weinberger said, thanks to the apparent willingness of the Israeli government to withdraw troops and the recent peacekeeping efforts of Secretary of State George P. Shultz.

Weinberger also was on a bit of a peacekeeping mission in coming to address the 40,000-member American Jewish Committee. He sought to convince the audience that he was not anti-Israel, a charge made by about 200 demonstrators lined up across the street from the Hilton Hotel, where the defense secretary made his luncheon speech.

"Cap Knifes Israel" read one sign carried by the demonstrators. "Stop Appeasing Soviets," read another. The National Council of Young Israel passed out flyers calling Weinberger the "architect of the Reagan administration's anti-Israel policies."

Weinberger attacked the issue head-on about halfway through his speech.

"Let me stop at this point and talk about something which has concerned both of us deeply," he said. "And that is the allegation that I, personally, have some animus against Israel. I want to say, as forcefully as I can, that this is simply not true. I am a strong supporter of Israel and an admiring witness to the democracy they have built and preserved under the most trying conditions . . . .

"Even if the American people were not bound to Israel by emotional ties, as secretary of defense, I would still be a strong supporter of Israel. Leaving all sentiments aside, looking only at our own national interests, it is clear that we in the United States have an important stake in Israel's security.

"We know that the Soviets would dearly love control over the Middle East's resources and strategic choke points but Israel stands determinedly in their way."

His emphasis on Israel's strategic importance to the United States drew one of the four rounds of applause he received.

After he sat down, Maynard I. Wishner of Chicago, national president of the committee, turned to the defense secretary and asked, "Do you want another strudel?" This seemed to break the tension between Weinberger and his audience as waves of laughter washed through the hotel ballroom.

Turning serious, Wishner said that the Jewish organization was grateful "that your door has been opened." Wishner later told a news conference that he "felt reassured" by Weinberger's words.

Howard I. Friedman, the newly elected president of the American Jewish Committee, said that he, too, felt reassured. He said the defense secretary had spoken "against a backdrop of anxiety in the American Jewish community."

In answering questions after his address, Weinberger made these points:

* Memorandum of understanding. He noted that the document that he and his Israeli counterpart had signed outlining mutual interest and concerns about the strategic situation in the Middle East had been suspended with the Israeli invasion of Lebanon. But Weinberger said that he saw no reason now that it could not be revived.

* Lavi fighter. Israel has received almost all the American technology it has requested for building the fighter, which is to be sold to other countries. Weinberger added that the one bit of technology which Israel has requested but not yet received is so advanced that no other country has been authorized to get it. Even so, Weinberger said, he believes the Reagan administration soon will let Israel use it in its new export fighter.