Ambassador Malcolm Toon, whose blunt-spoken ways ruffled Soviet and American diplomatic feathers alike during his nearly three years as the U.S. envoy to Moscow, ends his career here with a grim view of Soviet expansionism and sharp criticism of Washington for bypassing his embassy in some key negotiations.

In an interview here yesterday the 63-year-old career diplomat asserted that Americans must support higher defense spending virtually across the board to brake Soviet power around the world and ensure strategic and conventional arms parity with the Soviet Union in the 1980s and beyond.

He reiterated his own backing for the SALT II treaty, warning that U.s. Senate rejection would lead to "rather substantial deterioration of our relations."

In assessing his tenure here, which began under strained circumstances Dec. 30, 1976, and ends when he departs Tuesday Toon declared he is generally pleased with both the present state of relations between the superpowers and his contributions to that.

Nevertheless, he is leaving the Soviet Union and the Foreign Service with his original perceptions of what he sometimes privately calls "this benighted nation" largely intact.

"You can be sure [the Soviets] are going to be watching very carefully what happens to Washington over the next decade," he said, "and if they feel that we're becoming flabby, inconstant, weak in our determinaton to protect our interests then you can be sure that they are going to move, in some cases in a dangerous way. That is way I think it's really terribly important to peace and stability in the world that we behave in such a way that everybody has the perception that we remain a strong power determined to protect the vital interests of the free world."

But he saved his sharpest public words as he has frequently in private, for the distinctly secondary importance of the U.S. Embassey here in formulating and implementing U.S. policy toward the Soviet Union.

A fluent Russian speaker, Toon saw his initial hopes of gaining wide access to the ruling Politboro frustrated both by traditional Soviet secretiveness and lack of support from Washington. As a result, he said, Soviet perceptions of America at the highest Kremlin levels continue to suffer from isolation, with attendant troubles in the bilateral relationship.

He said that on several key matters, the embassey was shut out by the Carter administration as it negotiated with the Soviets, and he observed that subsequent difficulties might have been avoided if the -- and he staff -- had been included.

For example, he acknowledged that even now he "still is not privy" to details of the negotations that led up the April swap of two Soviet spies for five imprisoned Soviet dissidents and their families. The family of one, Alexander Ginzburg, is still have struggling to gain freedom for a young man the family had adopted unofficially.

While asserting that "in general, the exchange went well," Toon observed some of these loose ends were just left dangling. You shouldn't do that sort of thing when you're dealing with the Soviets, and if in fact this is what happened, then they could have used, with considerable profit, advice from the embassy. . . and perhaps could have profited by actual participation by this embassy in the negotiating process".

Toon's relations with Washington have been marked by the same fluctuations that have characterized his relations with the Soviets.

When President Ford appointed him shortly before the 1976 election, the Soviets tried to prevent the appointment by delaying approval. President Carter subsequently withdrew, then reinstated, the appointment.

After the March 1977 episode in which the Soviets bitterly rejected the Carter administration's initial, sweeping proposals for major mutual strategic missile reductions. Toon was brought in as a key adviser. But by the end of the negotiating process last May, he was no longer playing an active role. He said he was "unhappy" about this, but added that it was "understandable that !Secretary of State Cyrus R.) Vance and others in washington felt in order to avoid leaks it was wise to confine the negotiations to one channel, the Washington channel."

He called the treaty "the major achievement" of his tenure here. "I played a modest role in the negotiating process. I had a considerable input to the process of formulating our positions and therefore I think I can look back with a small degree of pride on my participation in this concluding what I think is a very important treaty.

His major frustration, he said, was the unequal treatment accorded his opposite number, Soviet Ambassador Anatoliy Dobrynin, by Washington. Where Toon has been consistently shut out of the Kremlin's higher circles, Dobrynin has long had wide contacts with every administration and Congress in Washington for the past 17 years.

It's a very difficult problem to grapple with," Toon said, "because you have (there) a very able ambassador, one for whom I have great adimiration although I disagree with almost everything he does and says . . .He knows everybody in town.

"He perhaps has known the American scene a little bit better than most of the people that are handling the levers of power today. So it's difficult to get around this problem, because you are dealing with an ambassador with broad experience, great ability, considerable charm. An ambassador with a talent for dissembiling, shall we say?

"Perhaps he is trusted more than he should be. I think people back home should never forget that the Soviet ambassador, no matter where he may be, is a tool, as instrument of Soviet policy, and he himself has very little power and is operating on the basis of strict instructions from the Politburo. And those instructions are formulated by the men who have very little perception as to what the outside world is like."

Toon has had several formal meetings with Brezhnev and numerous sessions with Foreign Minister Andrei Gronyko as well as with various younger, alternate members of the Poliburo. But these have been few and far between over his two years and 10 months here. He said he has "shaken hands" with almost all 13 Politboro members but done little more. It is the same treatment accorded virtually every foreign envoy here.

Toon said the lack of access "that improves mutual understanding" cannot be overcome as long as Dubrymin can more freely and the American ambassador here cannot.

"I think there continues to be a lack of understanding on [the Politburo's ] part as to what we're up to, what our objectives are, what we stand for. . .

We do have difference [between Toon and the State Department] -- rather serious differences -- as to tactics, in how you handle the Soviets. This gets into how much you use this end and how much you rely on the Washington channel.

"My own feeling, strongly held, is that this heavy reliance on dealing with the Soviet Embassy in Washington down through the years is an unhealthy development and I would hope we recongnize this. . .

"When you have something to say to the Soviet leadership, you say it through your embassy, your ambassador. I think this a primarily useful when you have an ambassador here who speaks the language, as we have had traditionally up until now. When you have an embassy that understands, insofar as it is possible for any Westerner or Western institution to understand, the Soviet system, I think you should speak through your embassey to make sure there is no misunderstanding at this end as to what our policy and attitudes are."

Toon is one of the vanishing breed of senior diplomats who served in World War II (he was a decorated PTboat commander in the South Pacific) and later became Sovietologists.

". . . When you could see victory at the end of the road. I began to think about the postwar world and it seemed clear to all of us that the real threat on the horizon was Moscow, and therefore I thought I ought to do what I could to gear myself to deal with the problem."

He learned Russian and served in various posts in Eastern Europe and twice here before being appointed by President Nixon ambassador to Yugoslavia, Czechoslovakia and Israel.

His successor, retired IBM Chairman Thomas J. Watson Jr., arrives here Wednesday as America's 16th ambassador to the Soviet Union.