Chancellor Helmut Schmidt today termed "astonishing" President Carter's letter to him last week that called into question the West German leader's position on new nuclear missles in Europe.

In an hour-long interview, Schmidt strongly defended his recent proposal that East and West agree to postpone deployment of medium-range missiles for three years as being in line with previous NATO decisions and "in the mainstream of Western thinking."

The chancellor said he expected the subject to come up in his talks with Soviet President Leonid Brezhnev in Moscow June 30. It was concern about the Schmidt-Brezhnev meeting that apparently prompted Carter's letter. aIts content, timing and the apparent leaking of its contents from Washington have offended West German officials.

Schmidt said he was concerned about an imbalance in such weapons in Europe and would not acquiesce to it. But, he said, while West Germanyy's national interests include a close alliance with its Western partners, "we indeed will also not give up the will to cooperate with" the Soviet Bloc countries.

During the interview, Schmidt stressed his role as initiator and architect of NATO's decision last December to produce and deploy 572 medium-range nuclear missiles in Europe to offset a buildup in Soviet weapons. Two months ago, Schmidt publicly floated the controversial three-year delay proposal. The German chancellor said today that he "found it difficult to understand" why this suggestion "should have created such fuss in Western circles."

At the core of the apparent disagreement between Schmidt and Carter is a question of strategy on how to deal with the Soviets. The deployment of 572 U.S. Pershing II and cruise missiles is not scheduled before 1983 in any case. Schmidt's initiative appears to have the advantage of stopping the current deployment of Soviet SS20 missiles, in exchange for which NATO would negotiate about its own mnissile deployment.

The fact that Schmidt's proposal has become so controversial is an indication of Western concern over current relations with the Soviets in general, and so-called theater nuclear forces in Europe in particular, as well as the degree of Western unity.

"It is a false conception," the chancellor said, "to believe European governments don't have a right to voice their concern, don't have a right to make their proposals."

Schmidt declined to disclose what he said in his reply message to Carter this week, but the two leaders will be conferring in Venice Saturday night on the eve of the summit meeting of the heads of the West's major powers.

On other points in a wide-ranging talk, the 61-year-old chancellor of Europe's economically most powerful nation:

Excused current strains in the Western alliance as "not a new phenomenon."

Said that he does not believe the West should try to "punish" the Soviets because that is not something countries should try to do to one another. But he again called the Soviet intervention in Afghanistan "unacceptable" and said the Western alliance is agreed the Soviets must withdraw.

Spoke of the importance for West Germany of maintaining "normal relations" with the Soviet Union.

While commentators on both sides of the Atlantic have been saying that U.S.-European relations have reached a low point in recent weeks. Schmidt blamed all the "nervous comments" on press reports and on highly-charged political atmospheres resulting from national election campaigns in the United States, West Germany and Italy. Recalling past disputes, Schmidt said disagreements have always come up within the alliance.

What bothers him, he said, is not the state of the alliance but the expansion of Soviet political and military power over the past 20 or 30 years -- particularly Soviet superiority in nuclear forces in Europe.

"The imbalance [in East-West nuclear weapons in Europe] has great importance, more political importance than military," said the chancellor. "There is a political potential behind or implied in such an imbalance."

To catch up, NATO ministers voted last December to produce and deploy U.S. Pershing II and cruise missiles at U.S. bases in several Western European countries.

At the same time, the NATO ministers offered to begin negotiations with the Soviet Union to limit the number of such weapons. But the Soviets, who have already begun to deploy new SS20 mobile, multiple-warhead nuclear missiles targeted on Western Europe, have rejected any negotiations unless NATO renounces its decision to go ahead with its new missiles.

Schmidt, looking for a way to break the impasse, proposed in April that both sides agree not to deploy their weapons for three years, but instead begin negotiations on mutual limitations.

The surprise proposal at first caused considerable confusion in Western circles. It was interpreted by some as a softening of West Germany's support of the NATO decision. Schmidt forcefully denounced this notion.

NATO officials worried meantime that by agreeing not to deploy temporarily, they might be barred from going ahead with extensive preparations needed for the new missiles. Schmidt said today his offer never implied this. n

However, NATO officials also fear that action on Schmidt's proposal would require a second alliance decision that might undo the carefully crafted first one.

This doubt and confusion clearly disturb the chancellor. "It was I who brought the decision-making process in the first place," Schmidt said in recalling his London speech in 1977 in which he called attention to the growing Soviet nuclear threat in Europe. "Why should I abandon it. This has been my writing and speaking for 20 years. Some people who have come to the scene rather later than I don't seem to know all the history behind it."

Schmidt floated his latest proposal originally in a political speech in Hamburg. He gave it, as he said today, "off the cuff" without consulting other alliance members.

Asked why he chose to act independently, the chancellor answered, "It has been my habit for 20 years to voice my ideas without asking anybody else. I was and am still sure I was acting not only in the mainstream of Western thinking, but I do also influence Western thinking quite a bit."

The chancellor expects to discuss the Moscow visit during the summit meeting with Carter and the leaders of France, Britain, Italy, Canada and Japan. He has said that when he goes to Moscow, he would follow a line agreed on after consulting with West Germany's allies.

Two things Schmidt said he intends to tell the Soviets is that their military intervention in Afghanistan is unacceptable and must be ended, and that West Germany and its alliance partners will not acquiesce to the current imbalance of nuclear forces in Europe.

But indicating the tack he will suggest to Moscow, Schmidt added it would be better to restore a balance of forces through arms control and disarmament talks than by an arms race.

In dealing with the Soviets, it was clear the chancellor favored negotiation over confrontation. Questioned on whether he thought the Soviet Union should be "punished" for the Afghanistan invasion -- the view taken in the United States -- Schmidt said "I never liked that wording. I don't think that states should try to punish each other. I think the military intervention in Afghanistan is unacceptable, but I do not believe you get a reversal by punishment."

Discussing Bonn's relations with Mowcow -- which have become increasingly relaxed during the past decade -- Schmidt said he intends "to pursue normal relations with the Soviet Union, which implies cooperation in some fields."

Earlier this month, West German and Soviet officials agreed to a protocol for a 25-year economic cooperation accord between their countries, and the final signing is expected to take place during Schmidt's Moscow visit.

The chancellor noted that Bonn is not alone in seeking continued cooperation with the Kremlin. The United States, he said, is still conferring with the Soviet Union on ocean rights, and Carter "still thinks of ratifying Salt II."

But beyond that, Schmidt explained that West Germany's relations with the Soviet Union and Eastern Europe mean something more, noting the strong historical and human links between this nation of 60 million people and its communist neighbors. "It is an historic and moral necessity" for Bonn to maintain its Eastern policy, the chancellor said.

At the same time, Schmidt said in answer to some critics in Washington, "this is not a position of weakness. It has the backbone of equilibrium and the close alliance with the West."