Defense Secretary Caspar W. Weinberger said yesterday that a new Soviet arms proposal in Geneva that would prevent either the United States or the Soviet Union from withdrawing from the 1972 Antiballistic Missile Treaty for 20 years was "terribly wrong . . . and against [the U.S.] national interest."

Until Weinberger's remarks, administration spokesmen had refused to comment on the Soviet offer, which was presented in Geneva last Thursday by the chief Soviet delegate, Victor Karpov. State Department officials said yesterday that a U.S. position has not yet been formulated and that the matter was still under discussion in Geneva.

Weinberger, during an interview on CBS "Nightwatch", said the Soviets had proposed cancellation of the treaty's withdrawal clause that permits either nation to abrogate the accord with six months notice. The treaty has no time limitation, but it calls for a review of its terms every five years. The next review is scheduled for 1987.

Weinberger said the Soviets had also proposed "to strengthen" the treaty's provisions.

He called the offer a "side door" to killing President Reagan's Strategic Defense Initiative (SDI), the so-called "Star Wars" research program for a nuclear missile defense.

Weinberger said the Soviets knew that if the administration agreed not to deploy an SDI system for 10 to 20 years, Congress would refuse to fund even the research. "The Soviets know you can't get funding for a program if you've said you're not going to use it for 10 years."

Such an agreement, he added, would cause SDI research "to lose a great deal of momentum" and cause a "loss of all public support or the possibility of ever deploying a strategic defense." Privately, some other officials have said the Soviet offer was promising, depending on its details. It appeared to echo a proposal to extend the time between notice of withdrawal and actual abrogation that had been discussed years ago in the State Department under then-Assistant Secretary of State Richard R. Burt. An arms control expert, Burt is now U.S. Ambassador to West Germany.

Weinberger also said yesterday that he was "surprised at the amount of discussion" generated by the president's announcement last week that he would no longer be bound by the limits of the unratified SALT II treaty. Asked about criticism from NATO allies, Weinberger said the Europeans believe "some kind of agreement is better than none."

Asked why the United States decided to drop the entire SALT II agreement rather than just violate the same provisions it accused the Soviets of violating, Weinberger said, "We don't want to respond to a violation with a violation."

At a House Armed Services subcommittee hearing yesterday, Edward L. Rowny said the president's decision would eventually break the deadlock at Geneva.

Rowny, a former U.S. strategic arms negotiator who serves as Reagan's special arms adviser, said, "Now the air is clear to concentrate on what the president wanted to do in the beginning: Negotiate deep reductions. This is a new starting point. We have a way to get the Soviets to concentrate on what arms control should be all about."