A major West German poltical figure, just back from a controversial visit to Moscow where he met with Soviet President Leonid Brezhnev, said yesterday that the Soviets were unmistakeably preparing new arms control proposals and seemed intent on giving a boost to the stagnating policy of detente.

The German official is Egon Bahr, the general manager of the ruling Social Democratic Party. Though not an elected official, Bahr is a former top aide to former West German chancellor Willy Brandt and was the architect of Brandt's dramatic policy of "ostpolitik" - the effort by West Germany in the 1960s to restore relations with its wartime enemies in Eastern Europe.

Bahr, however, is also a respected and important figure on the left wing of Chancellor Helmut Schmidt's party, and it was his description last summer of the U.S. developed neutron warhead as an example of "oreverted thinking" that became the emotional rallying cry for virtually all European critics of the weapon.

Thus, while Bahr, has probably better contacts with the Kremlin than any other major West German - and perhaps Western - figure except possibly Brandt these days, his trip at this time - just a few weeks before Brezhnev is due to visit Bonn - has ruffled some feathers here.

Bahr's mission to Moscow was actually for Brandt, who is head of the newly created North-South Commission aimed at improving understanding between rich industrialized nations and less developed countries. In that sense, his trip was unofficial as far as the Bonn government was concerned.

But Bahr, who met with Chancellor Schmidt before he left and again yesterday, also spent time with both Brezhnev and Boris Ponomaryev, Secretary of the Soviet Communist Party's Central Committee, discussing the forthcoming meeting between the Soviet leader and Schmidt in Bonn next month.