Soviet President Leonid Brezhnev is to arrive in Bonn Sunday for what is seen here as one of the most important visits he has made in years.

West Germany is not only a major Soviet trading partner, but is also the only NATO country with which Moscow remains on good speaking terms. Its chancellor, Helmut Schmidt, carries weight not only in West European capitals, but in Washington as well.

But the main function of Brezhnev's journey, apart from trying to cement this relationship, appears to be a search for political advantages in the propaganda battle surrounding the talks on medium-range nuclear weapons, which start in Geneva Nov. 30.

In West Germany, which is to receive the largest number of these new weapons, the campaign against their deployment has put Schmidt's coalition government under strain. Similar pressures are at work in Belgium and the Netherlands, where 350,000 people protested against the missile deployment today.

Aided by controversial remarks by President Reagan and antinuclear sentiments in Western Europe, the Soviets appear to have made some headway in their efforts to block a 1979 NATO decision to deploy a new generation of American medium-range nuclear missiles in Western Europe.

With Reagan's arms-control proposals last Wednesday, however, Moscow appears to have lost its momentum. Hence the foray of the 75-year-old Soviet leader to Western Europe is given added importance. The Bonn visit, said a Brezhnev spokesman, is an "important factor not only in bilateral relations but also in European politics as a whole."

Some observers in Moscow suggest that Brezhnev may attempt to regain the initiative during his Bonn visit by offering some substantive Soviet concessions.

Whether this is correct was impossible to ascertain. But given the fact that the Soviets regard West Germany as the key to Western Europe, it is likely that Brezhnev will come forward with proposals designed to demonstrate Soviet sincerity about arms control while at the same time raising European doubts about the continuity and rationality of U.S. nuclear policies since Reagan took office.

As usual before Brezhnev's trips abroad, the press here has blossomed with reports on Soviet-West German trade and other benefits of detente. But these reports are somewhat overshadowed by a massive propaganda drive that centers on Reagan's remarks about a limited nuclear exchange in Europe and Secretary of State Alexander M. Haig Jr.'s statement about NATO plans for a demonstrative use of nuclear weapons in case of a conventional Soviet attack on Western Europe.

An authoritative commentary in the Communist Party newspaper Pravda today again accused the Reagan administration of trying to gain strategic superiority. It dismissed Reagan's arms control proposals as "hypocritical" and designed to "deceive the public."

"In reality, the United States is imposing on Europe a new, ever more dangerous spiral of the nuclear arms race leading to the aggravation of military confrontation fraught with creating a crisis situation on the European continent," the commentary said. It was signed by Alexei Petrov, a pseudonym used for high-level pronouncements.

A senior Soviet official, however, said in a news conference yesterday that Reagan's speech on arms control was a welcome change from his earlier statements.

"Since entering the White House, Reagan has spoken in a very bellicose way . . . ," said Vadim V. Zagladin, a member of the Communist Party's Central Committee. "If, in fact, he now wants to be a peacemaker, then we can welcome this as a turn for the better."

In a long interview with the West German magazine Der Spiegel recently, Brezhnev asserted that "if nuclear war breaks out, whether in Europe or in any other place, it would inevitably assume a worldwide character."

The statement reflects Soviet nuclear doctrine and was presumably made to disabuse Reagan of any illusion that the United States would be spared in a nuclear exchange. But it also played on the fears of Europeans.

Although Brezhnev's Bonn trip has a larger purpose, one should not underestimate the importance of bilateral ties. This will be Brezhnev's first visit to a Western country since the invasion of Afghanistan in December 1979, marking an end to Moscow's isolation. Moreover, Soviet-West German trade has continued to grow despite the East-West chill caused by the invasion.

Today, West Germany is Moscow's main trading partner in the noncommunist world with total exchanges last year in excess of $8 billion. Ten years earlier, the total volume of annual trade was $770 million.

The two countries have been negotiating new and even larger joint ventures including the construction of a 3,000-mile natural gas pipeline from western Siberia to Western Europe. West Germany is to extend more than $4 billion in credits for that project alone.

Western critics have viewed the project as one that would make Western Europe dangerously dependent on Soviet energy supplies and thus politically vulnerable to Moscow.

But the project is also of enormous economic importance to the Soviet Union in its efforts to develop Siberia, where most of its natural resources are located.

Seen from here, the West German chancellor is not only Brezhnev's preferred contact among Western leaders but also an important interpreter of Western policies and intentions. Soviet spokesmen say the two men will talk "substance," presumably on the crucial issue of arms control.

Brezhnev is expected to have little trouble persuading Schmidt that Moscow wants to negotiate. Detente has been the cornerstone of Brezhnev's foreign policy. Under his leadership, the Soviet Union has improved its military strength and foreign influence.

At the same time, however, the Soviets have confronted limits on their power in their move into Afghanistan. In Poland, Moscow's imperial system is being severely tested. China remains an implacable adversary. These and other foreign policy problems are topped by economic weaknesses at home and new economic reforms.