The Pentagon said yesterday that Defense Secretary Caspar W. Weinberger has has taken the unusual step of writing editors around the world in hopes of convincing them that the Reagan administration is not planning to fight a nuclear war.

He complains in the form letters mailed Monday to 30 editors in this country and 40 abroad, primarily in NATO countries that are protesting putting additional American nuclear weapons on their soil, that the administration's nuclear policy has been misrepresented in the press.

One Weinberger associate said the letter was part of the secretary's effort to focus attention on one subject at a time and communicate the administration's views as directly as possible, rather than risk being deflected or misinterpreted in wide ranging news conferences.

Defense Department recored show that Weinberger has held only one general news conference at the Pentagon in his 19 months there, but has appeared on television in the United States 43 times in the same period.

His dislike for the wide open news conference was underscored Friday. President Reagan and Secretary of State George P. Schultz appeared before reporters to announce that 800 Marines were going into Lebanon, but Weinberger left his department's briefing to spokesman Henry E. Catto Jr.

"This is the kind of nuts-and-bolts thing that I don't think is appropriate for him to have to worry about," Catto said when asked Friday why Weinberger "didn't hold this briefing."

However, Weinberger appeared on TV talk shows Sunday and Monday to discuss the situation in Lebanon.

Catto said yesterday that Weinberger's letters on administration nuclear policy are designed to counter the printed misinterpretations of that policy, particularly in Europe "from people clearly on the left."

In his form letter, which has a "Dear Sir" salutation, Weinberger opens with: "I am increasingly concerned with news accounts that portray this administration as planning to wage protracted nuclear war-fighting capability.

"This is completely inaccurate, and these stories misrepresent the administration's policies to the American public and to our allies and adversaries abroad. It is the first and foremost goal of this administration to take every step to ensure that nuclear weapons are never used again, for we do not believe there could be any winners in a nuclear war."

Weinberger wrote that Reagan is strengthening the nation's nuclear forces as part of his basic strategy of keeping the Soviets from pushing the button by making "the cost of nuclear war much higher than any possible benefit to the country starting it . . . . There is nothing new about our policy."

To match the growing Soviet nuclear arsenal, Weinberger said that "we must have a capability for a survivable and enduring response, to demonstrate that our strategic forces could survive Soviet strikes over an extended period." The words "capability" and "could" are underlined.

The controversy Weinberger is trying to still stems from the way some newspapers interpreted his secret guidance to the military services. The guidance was obtained by The Washington Post and The New York Times. The Times article on May 30, which drew a Soviet protest, was headlined "First Strategy for Fighting a Long Nuclear War."

The guidance for fiscal 1984 through 1988 states that "should deterrence fail and strategic nuclear war with the U.S.S.R. occur, the United States must prevail and be able to force the Soviet Union to seek earliest termination of the hostilities on terms favorable to the United States.

This requires, among other things, nuclear forces that could hit the Soviet Union "throughout a protracted conflict period and afterward," according to the guidance.

Weinberger has complained repeatedly to congressional committees that the administration's policies have been misinterpreted by the press.

The Pentagon confirmed yesterday that the secretary's only wide open Pentagon news conference was on Feb. 3, 1981. Catto said yesterday that, considering how willing Weinberger has been to meet with reporters individually and to appear on television, as well hold news conferences outside the Pentagon, "his availability has been terrific."