The people of Washington got a real bang-up Easter surprise: a voice from the White House telling them that there is only a 40 percent chance of nuclear war.

It was not news, of course, that Richard N. Pipes, the Harvard professor who now serves Ronald Reagan on the National Security Council as an expert on the Soviets, believes that American liberals are exhibiting "uncontrolled anxiety" about the bomb and should learn from conservatives, and Soviet nonchalance, to be "more restrained" in their approach. The mystery is why anyone at the White House thought that having Pipes express his grisly thoughts would somehow bring credit to the president.

Reagan has been having trouble enough coping with manifestations of "uncontrolled anxiety"--the booming, broad-based nuclear freeze movement, calls for a U.S. "no-first-use" pledge and expressions of Republican disquiet. Rep. John J. Rhodes (R-Ariz.), no nervous Nellie, said the other day that "the United States and the Soviets are playing poker with sticks of dynamite."

But William P. Clark, the national security adviser, apparently thought the public would be reassured by a glimpse into the howling wilderness that is the mind of Richard Pipes, who was muzzled last year after he announced that the Soviets had a choice between change and war.

In the ensuing furor, no thought was given to dismissing him. It was explained that he was speaking "personally," the same rationale given in the case of the notorious T. K. Jones, the deputy undersecretary of defense, he of the two-to-four year recovery from nuclear war and the door-and-three-feet-of-dirt school of nuclear survival.

Pipes reviewed the quotations in the interview, published Sunday, and apparently found them good. He made no mention of arms reduction or even control. The only way to avoid a nuclear war is to build up our nuclear stockpile, so that our "deterrent" is credible to the crazy Soviets, who wouldn't mind starting one, he said.

For the first year in office, Reagan and Defense Secretary Caspar W. Weinberger always ducked the question of whether they thought the United States could win a nuclear war. The one thing about which they were absolutely emphatic was that they knew that the Soviets thought they could win a nuclear war.

"The point, I think," Weinberger said last Oct. 3, is not whether we think we can win a nuclear war . . . . What is important is that the Soviets think a nuclear war could be won."

Many people were puzzled by such certainty on this issue, since neither Reagan nor Weinberger has given any evidence of serious study of the Soviet character that would have led them to their conclusions about the depths of Soviet nuclear depravity. Pipes, plainly, has been their tutor.

The Soviets will not be deterred by the prospect that we will wipe out their cities, Pipes told his Washington Post interviewer. That is why the doctrine of mutual assured destruction is no longer in force.

What has replaced it is not clear. About all Pipes has to say is that in order to avoid nuclear warfare, we must prepare for it. Secretary of State Alexander M. Haig Jr. complained last week that this is a "caricature" of the strategy of deterrence.

But with the Reagan White House, it is always difficult to know when an official is speaking officially or "personally." Free speech is encouraged in foreign policy, although it is actionable on domestic questions, as Treasury Secretary Donald T. Regan learned when he suggested in December that the government needed "enhanced revenues."

On nuclear matters, anything goes. White House counselor Edwin Meese III, for instance, referred to nuclear war as "something that may not be desirable," and there was no trip for him to the White House woodshed.

Pipes contradicts Meese on civil defense. Meese endorsed the Federal Emergency Management Agency plan for mass evacuation of U.S. cities in the event of sufficient notice of attack. Pipes doesn't think it will work, although the administration has proposed a $4.3 billion expenditure on civil defense.

The Soviets are mad monsters who regard a nuclear war as just like any other, who contemplate coldly the destruction of their people and the ruination of the planet. But they are commendably sensible, because they don't see nuclear war as the end of the world, according to Pipes. And while he says in one place that they they are "extreme" in not fearing nuclear war, in another he says "the fear is just as great on the other side as it is on this side."

The hope is that Pipes is as wrong about the Soviets as he is about Americans. He fears for "human ability to deal with reality." His own grip doesn't seem to be too strong. He has not noticed that three-fourths of U.S. citizens favor an end to the arms race. And he was astonished to learn that 70 percent of today's undergraduates "expect to die in a nuclear war."

That number may rise when they hear the good news that he puts the chances of nuclear war at a mere 40 percent.