The Soviet Union poses grave dangers to world peace and stability that must be met by the United States whatever the cost, the U.S. ambassador to Moscow, Thomas Watson Jr. declared today in an interview on the eve of his retirement.

After 14 months as America's envoy here, in which his hopes of improving bilateral relations were shattered by the Soviet invasion of Afghanistan, Watson is convinced that the authoritarian Soviet government firmly embraces the expansionist aspirations of its Czarist predecessors, despite Kremlin protestations to the contrary.

Against this background, combined with the Soviet Union's proven ability to project power far from home and a continuing military buildup, Watson said, "I don't think the West has any conception of how dismal the future looks for East-West relations."

White-haired and soft-spoken, Watson looked somewhat haggard as he sat for an interview in his 9th-floor office at the U.S. Embassy. He seemed burdened by the job's demands, his dashed hopes and a sense of gloom about the Soviet challenge to America and the West.

The 67-year old ambassador said he believes there is "no hope" of any change in the Kremlin's global ambitions from the eventual successors to President Leonid Brezhnev.

"Lots of people say that when the postwar leadership comes along, people without parents who grew up in the revolution, without memories of World War II, then things will change. I don't think things will change . . . This is one of the most stable governments and unlikely-to-rebel people on the face of the earth. There is no hope of collapse, no hope of change.

"What there may be hope of a little better ability on the part of the West to understand them, and perhaps a little better ability on their part to understand us.But I don't have a very high hope on that."

A friend and admirer of former secretary of state Cyrus Vance for some years, Watson was chosen by Vance to succeed tough-talking Malcolm Toon as ambassador in October 1979.

A former president and chairman of IBM, Watson spent his time here buffeted by one of the bitterest bilateral periods in the postwar era. Although he had no diplomatic experience and could not speak Russian, Watson was stationed here for six months during the war as a pilot and had made numerous trips here since graduating from Brown University in 1937. On arrival as ambassador, he had wanted to concentrate on strategic arms limitation, which he had worked on in various citizen advisory capacities earlier. But the Afghan intervention in December 1979 overshadowed all other issues.

"I was surprised by Afghanistan, because I thought they had a better appreciation of the [dangers] of thermonuclear [confrontation] and I didn't think they'd be willing to take that kind of risk. [Perhaps] from their point of view, there wasn't much risk, that we thought it to be a relatively unimportant area and [their move] wouldn't distrub us greatly."

However, Watson said, the stiff American response, including the Olympic boycott, grain embargo, trade sanctions and closing of the U.S. Consulate in Kiev "may have made the world a litle safer by letting them know precisely what we would and would not tolerate."

Watson sees a long, painful time ahead for the United States dominated by the need for a strengthened military posture and credibility that he believes require the draft, higher taxes, energy belt-tightening and greater conventional arms spending. While maintaining strategic equality, America with its Western allies must also match Soviet conventional power, he said, adding:

"We must stay equal to the Soviets, project our power as we see fit to combat them around the world . . . We have no choice but to match them."