Defense Secretary Casper W. Weinberger said yesterday he believes the United States will have to begin deploying intermediate-range nuclear missiles in western Europe before the Soviet Union will negotiate seriously on mutual reduction of such missiles.

The prediction, which Weinberger voiced during a dinner at the U.S. Embassy in Oslo Saturday and repeated yesterday during his trip home, was more bluntly pessimistic than his assessments during the rest of his week-long trip through Europe. Many leaders there, facing popular sentiment against accepting the sophisticated new missiles in their countries, were anxious for some indication that Weinberger still believed an agreement could be reached before missiles start going in place at the end of the year.

"If we didn't put them in, we wouldn't get any kind of meaningful negotiations," Weinberger said yesterday. He added, however, that he nonetheless would not characterize deployment as inevitable. "I hope I'm wrong," he said.

Weinberger also argued in favor of insisting on throw weight as a measure of strength during negotiations with the Soviets on longer-range intercontinental nuclear missiles, the strategic arms reduction talks (START) that are about to resume in Geneva. Some U.S. officials are arguing against that position because they believe the Soviets are almost certain to reject it, thus making any agreement unlikely.

Weinberger agreed that insisting on throw weight could make the talks more difficult. Throw weight is the total lifting power available for nuclear missiles on both sides, a more complex category than simply counting the missiles and one in which the Soviets have a more commanding lead. He said throw weight is a fairer indication of war-making power.

"The question is do you simply want something, a piece of paper, that both parties can sign, or do you want something that can improve the situation," Weinberger said. "I don't think it's going to be easy at all, but that's the price we pay for letting that kind of imbalance open up."

Weinberger returned yesterday from a seven-day trip to West Germany, Belgium and Norway, during which U.S. nuclear policy and European concerns about it dominated many of his discussions with NATO leaders and foreign journalists.

The secretary will speak at the 45th reunion of his Harvard University class tonight and then return to Washington for a National Security Council meeting Tuesday, when President Reagan is expected to adopt a new negotiating position for the START talks.

The United States is planning to put 572 intermediate-range missiles in five west European nations in what NATO says is a response to Soviet deployment of nuclear SS20 missiles. The American weapons would include 108 Pershing IIs, based in Germany and capable of reaching Soviet targets in minutes, and 464 slower cruise missiles, which are unmanned low-flying airplanes.

Weinberger won a declaration of continued support from most NATO countries for the missile plan during his trip, but opposition parties and anti-nuclear groups in several northern European countries have been pressing hard to keep the missiles out of Europe.

Apparently recognizing the delicate political problems those governments face because of the opposition, Weinberger refused to predict during most of his trip when an agreement might be reached. One of his aides predicted a settlement in the fall.

But during his brief speech at the home of U.S. Ambassador to Norway Mark Evans Austad, Weinberger was more openly pessimistic. He also rejected the "walk in the woods" formulation, a possible settlement that was sketched out by the two nations' chief negotiators, Paul H. Nitze and Yuli A. Kvitsinsky, during a break in the formal bargaining last year.

That outline of a solution has aroused considerable interest in Europe, and a new congressional report here calls it "perhaps the best chance" for a settlement of the missile question.

But Weinberger dismissed the suggested solution, which would have allowed the United States 300 cruise missiles but none of the Pershings, saying it never reached the level of a proposal. He also said the idea was rejected by Moscow before Washington even learned of it.

"It's obviously something they rejected immediately because it would not leave them with their monopoly," Weinberger said.