Retired computer industry magnate Thomas J. Watson Jr., officially named yesterday by President Carter as the next American ambassador to the Soviet Union, speaks with candor about himself and some curiosity about his future as a novice diplomat heading the nation's most sensitive foreign post.

"I don't have an exalted opinion of my abilities," the retired chairman and president of International Business Machines Corp., said in an interview last month. "I'm no spring chicken . . . but at 65, I wouldn't accept the job unless the local ambassador could make a difference."

Watson gave the interview on condition that it would not be published until after his formal nomination. At the time, he was in Vienna, waiting in a hotel room during the U.S.-Soviet SALT II summit in expectation that President Carter would announce his nomination then. Carter waited until yesterday, apparently because of signs of political opposition to the naming of a businessman to the Moscow post, traditionally headed by a career diplomat.

Watson spoke hopefully of what me may achieve as successor to the present ambassador, career Foreign Service officer Malcolm E. Toon, whose blunt-spoken manner has ruffled Soviet feathers during his 2 1/2 years here as chief of mission.

"I have no prejudices about the Soviet Union," said Watson. "The Soviets and the U.S. have to have an atmosphere where they don't surprise each other . . . We tend to misunderstand them and they us, a synergistic process, and we get to misunderstand more than the facts would support."

As president of IBM for 19 years and chairman for 10 before retiring in 1971 after a heart attack, Watson, son of the firm's founder, presided over its leap to world dominance as a producer and a leaser of computers.

A multimillionaire with vacation homes in New Hampshire and the Caribbean, Watson has virtually no experience in international diplomacy. But his company's worldwide operations have carried him around the globe on sales trips and business deals.

He is an inactive member of the prestigious private Council of Foreign Relations, and a close friend of Secretary of State Cyrus Vance and noncareer diplomat W. Averell Harriman. Harriman strongly supported the idea of a businessman in the Moscow Post to improve relations. He had served here from 1943 to 1946 as ambassador, is remebered fondly by many Soviets as a man whose firmness of purpose was mixed with suave diplomacy.

Part of Harriman's success was attributed to his having had significant business deals earlier with the Soviets, struck a few years after the revolution. Watson has no such experience with the Soviets. He has visited here several times on brief business matters but says he remembers the Soviet Union best from a five-month military stint here in 1942 when he piloted U.S. lend-lease officers around the country in a four-engined B24 Liberator bomber.

"That time is terribly important to me, the greatest adventure of my life," he said.

Watson said he enjoyed using his rudimentary Russian for man-in-the-street interviews then in the threatened wartime capital and wondered if the Soviets would allow him similar freedom as ambassador.

A physical fitness enthusiast who skis, mountain-climbs and sails, the ambassador-designate also wondered if he and Olive, his wife of 38 years, could ride their bikes around the city for exercise. He seemed unhappy to be told that Moscow's robust traffic frequently threatens even pedestrians who keep to the sidewalks.

"Well, can the ambassador just take a walk around by himself?" Watson asked.

Of U.S. global policies, Watson mused, "I think it would be very nice for the United States if all countries of the world would be democratic."

Of Soviet policies, he commended, "I think the Soviet Union thinks [it has] a good system, more recently thought up, and [it] would like to get a lot of people to go in that direction. I don't want to equate the way we and the Soviets reach those goals. They're totally dissimilar."

He added, "It's difficult to understand the Soviet Union, they so stridently pursue their goals and on top of that, there is the thermonuclear umbrella . . . unless you learn you can't confront each other the way you used to, can't take the risk of each side making a wrong guess . . . if you happen to make one wrong guess, it can bring about serious consequences."

"Brezhev has said there can be no ideological detente. I tend to agree with him [because] we'd have to give up what the founding fathers did."

Watson contributed what he says is a modest sum to Carter's election campaign - "I don't actually remember, but isn't the personal limit a thousand dollars?" - and, with other Democratic businessmen, sent highlevel job recommendations to the president-elect in 1976.

Much of the diplomatic tension and the complication of life in Moscow for diplomats and other resident Americans seem distant from Watson's concerns during the interview. He asked whether it would be possible to plub his American hi-fi set into Soviet electricity as Spaso House, the official residence, and grinned when told it was possible with the use of a transformer.

An impeccable dresser, Watson had carefully linked Toon's name on his left hand to be sure of it.

The Soviets have agreed to accept him as the next enjoy, said Senate confirmation hearings are expected soon. Watson said in Vienna he hoped to arrive here in the fall. Toon, 62, who had sought to have a career diplomat replace him will retire and will speak on behalf on the SALT II for the Carter administration CAPTION: Picture, THOMAS J. WATSON JR. . . . from IBM to diplomacy