Harold Brown, in less than two months as Secretary of Defense, has drawn an entirely different picture of the Soviet threat than his predecessors.

More significant, he has persuaded much of the Congress that his picture is the accurate one; that it is safe to cut the defense budget despite the alarms sounded last year.

Even such a seemingly unsinkable project as the Navy's giant Nimitz air-craft carrier has become politically vunerable between last year - when President Ford and Congress approved building another one - and this year - when Ford, President Carter and the House decided against the Nimitz.

Encouraged by the Congressional support so far, Brown is steaming ahead with even more controversial reductions, including cutting the Pentogon payroll, revamping the military pension system and closing bases in job-hungry congressional districts.

His civilian service secretaries, taking their cue from him, are refusing to sign off on building a number of weapons until they pass much tougher tests than those imposed last year. Brown has said the Ford administration bought too many weapons too fast, virtually guaranteeing that money will be wasted.

His view of the Soviets is nowhere as dark as that of James R. Schlesinger and Donald Rumsfeld, his immediate predecessors as Defense Secretary. Brown recently complained to aides, for example, that the scary briefing on the Soviet buildup that the Pentagon has been giving Congress tells only one side of the story.

The Kremlin, he argued, could put together an equally scary briefing about American weaponry.

Brown, who knows more about the technical aspects of weapons than any other Defense Secretary of the nuclear era, takes the longer, calmer view of the arms race, as illustrated by this recent statement to Congress:

"Generally speaking , there is no reason for immediate or grave alarm about our ability to deter major military actions by the Soviet Union . . . A comparions of United States with Soviet investments during the past 20 years will show that, cumulatively, we have made as large an effort as the Soviets. However, a major part of the U.S. effort came during the first 10 years, while the most significant Soviet investments have been made during the past decade. We have probably lived off our earlier investments longer than we should. We have some catching up to do."

The tenor of Brown's remarks contrasts with the one broadcast by Rumsfeld in his final posture statement sent to Congress in January: "The Soviets by their activites indicate that they are not interested in mutual assured destruction. Accordingly, they must by accepted for what they are, not for we want them to be. Their actions indicate that they take nuclear war seriously. The United States must do no less . . . The United States effort must be as serious, as steady and as sustained as that of the Soviet . . . Prudence requires that we take into account the other and darker face presented by the Soviet Union . . ."

Brown, by taking a much softer line than Rumsfeld and apparently getting the majority of Congress to accept it, is attempting to change the government's thinking about the Soviet threat. A military officer who deals with Congress agreed, but said Brown is riding the natural desire for reassurance.

"Nobody likes to go to bed and worry about getting blown up," he observed.

Some military officers who contend the Soviet threat has been under-stated rather than overstated in the past are concerned that the softer line coming from the top of the Pentagon may slown the American military buildup.

But Carter and Brown can, and do, point to the fact that the Pentagon budget is substantially higher this year than last, even after allowing for cut - $120.37 billion in fiscal 1978 compared with $110.2 billion in fiscal 1977, a 9.2 per cent increase.

The $10 billion hike makes it easier for politicians to support the $2,8 billion cut Carter and Brown recommended after reviewing Ford's defense budget for fiscal 1978. Also, law-makers can assure constituents by stating that the Joint Chiefs of Staff support the lower budget.

Carter and Brown got the chiefs aboard before submitting their proposed cuts to Congress.

"The total budget will provide adequate security," Air Force Gen. George S. Brown, chairman of the joint chiefs, told Congress when asked about the reduced defense budget.

After only two months it is clear that Carter and Brown share a philosophy about national defense with these tenets: the Soviet military threat has been exaggerated in the past and there is no need to jump into crash programs; haste has been making waste on a number of military research and production contracts; smaller can be better in weapons, with a half-sized aircrafts carrier and cruise missiles cases in point; the only way to achieve the $5 billion to $7 billion gross, but not net, reduction Carter promised in the Pentagon budget during the campaign is to cut people and real estate costs, meaning closing military bases.

It is clear where Carter and Brown want to go in defense but not whether Congress will allow them to get there.