Defense Secretary Caspar W. Weinberger said yesterday that it would be a mistake for the Reagan administration to rush into a new arms control agreement and that limiting the Strategic Defensive Initiative (SDI) in exchange for Soviet cuts in offensive nuclear weapons would be a "bad bargain."

Weinberger said he believes "that the Soviets want and need an arms reduction agreement." He said the Reagan administration should not behave as if "speed or just signing a piece of paper is the important thing. It's more important than ever that we pay attention to the content of the agreement rather that just having the process drive an agreement."

Weinberger's warnings came during a Pentagon interview with six Washington journalists which he used as a forum to oppose what some have called the "grand compromise" of trading substantial reductions in the superpowers' nuclear arsenals for a delay in the deployment of any U.S. missile defense system.

The Soviets have proposed deep cuts in strategic nuclear arms in return for a U.S. promise of continued adherence to the 1972 Antiballistic Missile (ABM) Treaty for another 15 to 20 years. President Reagan will respond to Soviet leader Mikhail Gorbachev in a letter that U.S. officials said will be sent within the next few days.

Senior officials said over the weekend that Reagan would offer to negotiate on all aspects of arms control. They said Reagan would reaffirm the U.S. intention to continue with research on a missile defense system under SDI but would be willing to discuss limitations on deployment, with some favoring five or six years as a U.S. counterproposal.

Weinberger declined to directly oppose such a counteroffer, which officials said has been favored by Secretary of State George P. Shultz. But when asked repeatedly whether a delay in deployment would be negotiable, Weinberger implied that he was unwilling to make such concessions.

"Anything that gives up strategic defense would not be worth it," Weinberger said. "It would be undesirable in every way."

Even without a new agreement, U.S. officials have told Congress, it would be at least six years before a missile defense could be developed and that deployment would take several additional years. As orginally proposed, the plan called for a defensive system to be deployed in stages, with the first stage intended to defend existing U.S. missile sites.

But Reagan has embraced the more ambitious idea of a defense he has called "my dream" of an antimissile shield that he says could protect the entire U.S. population. Supporting this concept in April testimony recently released by the House Appropriations Committee, Weinberger said that those who favored a defense that would protect only missile sites "don't understand the system and have not gotten the word."

Weinberger yesterday criticized the ABM Treaty, which was signed by President Richard M. Nixon, for abandoning the concept of strategic defense. Anything that would exchange "a promise as hopeful as strategic defense" for a Soviet promise to reduce its offensive arsenal is "a bad bargain for the world," he said.

The draft of the letter to Gorbachev was worked out during the past several weeks by a top-level group consisting of Reagan and his most senior foreign policy and defense advisers, according to a senior official. The top-level meetings, known only to a few people, supplanted the Senior Arms Control Group (SAC-G) which is normally the battleground for interagency debates over arms policy.

Weinberger, Shultz, White House Chief of Staff Donald T. Regan, national security affairs adviser John M. Poindexter, Director Kenneth L. Adelman of the Arms Control and Disarmament Agency and Adm. William J. Crowe Jr., chairman of the Joint Chiefs of Staff, were among participants in the meetings.

Special advisers Paul H. Nitze and Edward L. Rowny were dispatched last weekend to consult U.S. allies in Europe and Asia, respectively, about the U.S. letter. In addition, H. Allen Holmes, the State Department director of politico-military affairs, was sent to Europe to chair a special meeting of a NATO group dealing largely with policy toward intermediate-range missiles.

The report of these emissaries is expected by late this week, and officials said that Reagan's reply to Gorbachev could be on its way as early as next week.

Staff writers Don Oberdorfer and Walter Pincus contributed to this report.