The Soviet Union has so many problems, both military and economic, that the United States can safely adopt a "basically conservative" defense posture, Defense Secretary Harold Brown said yesterday.

In one of the calmest appraisals to come out of the Pentagon since the Eisinhower era, Brown cautioned against "acting as though we were engaged in a terminal arms race," and instead recommended a carefully paced modernization of American forces.

"I realize I have run counter to many widespread views about our military position relative to the Soviet Union," Brown said in the Pentagon's annual posture statement -- a document that this year is expected to influence how some senators would vote on the strategic arms limitation treaty (SALT) President Carter is championing.

But, Brown continued, "I think it is fair to say that my overall outlook coincided" with that of the nation's topranking military officer, Air Force Gen. David C. Jones, chairman of the Joint Chiefs of Staff.

To buttress that claim, Brown then quoted from Jones' posture statement, also released yesterday:

"There is too much pessimism about our currnet capability. I wouldn't swap our present military capability with that of the Soviet Union, nor would I want to trade the broader problems each country faces."

The White House is counting on Brown to take the lead in selling SALT II to the Senate. The administration also is hopeful that the joint chiefs will support the agreement, which is virtually ready for signing.

Brown went to unusual lengths in his 324-page posture statement to put a different light on "the-Russians-are-coming" interpretation of Soviet military might.

It is true, he said, that the Soviets seem to be spending from 25 to 45 percent more on their military than the United States is. But that does not necessarily mean they getting more bang for the buck, he continued.

"Bureaucratic inertia" stemming from the Soviets' military-industrial complex or "misplaced" fear of the North Atlantic Treaty Organization and China could explain why the Soviets have been increasing their defense budget by 3 to 5 percent "for more than 15 years," he said.

Getting into specific numbers, Brown acknowledged that the Soviets appear to have 45,000 tanks, cmpared with 10,000 for the United States. But, he said, such a "raw comparison does not convince me of Soviet military superiority in Central Europe or make it advisable for the United States to buy another 35,000 tanks."

A flat U.S. Soviet tank comparison fails to recognize the tanks NATO partners would deploy against a Warsaw Pact invasion, and leaves out NATO's 17,000 antitank launchers and 40,000 antitank missiles, Brown said.

He also said that the geographic disadvantages of the Soviet Union's navy and the technical shortcomings of its air force are often overlooked by those assessing the relative might of the superpowers.

The Soviet fleet is spread thin over several oceans, and much of it could be bottled up in a war, Brown said. Although the Soviet navy is improving, Brown continued, "there is little doubt" that the United States and its allies would prevail in the Mediterranean.

As for warplanes, Brown said the Soviets are improving, but added: "I must stress that Soviet avionics, munitions, pilot training and flying time do not approach U.s/. standards."

Brown, in noting that the United States has abandoned the Kennedy administration's effort to prepare enough military forces to fight two big wars and one small one, said, "We now recognize that a two-theater attack on our allies and forces has become increasingly implausible as a result of the deepening Sino-Soviet split and the improvement in our relations" with mainland China.

President Carter, like president Nixon before him, has decided to field enough military forces ot fight one big war and one small one -- the 1 1/2-war strategy.

Looking beyond the respective arsenals of the two superpowers, Brown said that the Soviet Union's total strength is being sapped by a stagnant economy and a decline in the working-age population.

Although Jones, in speaking for the joint chiefs, said he would not trade places today with his Soviet counterparts, the general also warned that "the days ahead my well be some of the most difficult we have ever faced... The military balance between the United States and Soviet Union is shifting adversely. We are inching closer to a potentially dangerous situation..."

In a separate foreign affairs section of his posture statement, Brown continued his generally upbeat tone, but warned that any Soviet intervention in Iran's turmoil "could well require a U.S. response."

The posture statements submitted to Congress yesterday by Brown and Jones are designed, in part, to justify the Pentagon's request for authority to obligate $135.5 billion in fiscal 1980 and spend $122.7 billion. The spending request is a 10.8 percent increase over fiscal 1979. Inflation will make the "real" increase about 3 percent, according to the Pentagon.

Brown noted that leaders in Washington and Moscow have concluded that strategic nuclear arms -- the doomsday weapons both powers have aimed at each other -- have limited diplomatic leverage.

As a consequence, he continued, the United States and the Soviet Union have turned to building up their nonnuclear forces after flirting with "new looks" featuring heavy reliance on nuclear weapons. Former Soviet premier Nikita S. Khrushchev followed President Eisenhower's lead in trying the "new look," Brown said. only to have his successors play catch-up by spending heavily on non-nuclear forces after he left office.

"Despite their vast nuclear superiority" over China, said Brown in discussing the muscle-bound nature of nuclear weaponry, "the Soviets have deemed it necessary to station as much as a quarter of their ground and tactical air forces in the vicinity of China."

The Pentagon's new budget earmarks almost 5 times as much for non-nuclear as for nuclear forces -- $50 billion compared with $11 billion.

Although skeptics may charge that Brown's posture statement was designed to improve the congressional climated for SALT II, the secretary has been skeptical of Russians-are-coming rhetoric since his first tour at the Pentagon in 1961, and has been calling ever since for cold analyses of how much is enough for defense.

"We must not allow the Soviets to believe that they can adopt adventurous and aggressive behavior in areas where the stakes are high," Brown said yesterday. "But we must avoid acting as though we were engaged in a terminal arms race. Our posture can be basically conservative in nature, designed both to control Soviet actions and to hedge against the main uncertainties of the future."