While West Germany and the Soviet Union trumpeted their new 25-year economic agreement signed here last week, a quieter but also politically important trade link has evolved between Bonn and the Kremlin's arch-rival, China.

With little fanfare, West Germany has become Chinas third largest trading partner after Japan and Hong Kong.

For Peking, the links to Bonn mean not only West German help in modernizing Chinese industry, but also a chance to keep whispering warnings into the ears of West German Leaders about Soviet military and political intentions in Europe.

For West Germany's export-oriented economy, the relationship means good business. Exports to Peking - including prefabricated chemical plants, steel piping, machine tools and electronic technology - tripled between 1972 and 1976, reaching an annual level of about $650 million.

Last year, however, West German exports to China dropped by about 25 percent, a situation that Bonn economic officials believe was due partly to the fact that several major plant construction projects were completed, and partly to the turmoil in the political leadership in Peking.

Last month, China's foreign trade minister, Li Chiang, made a seven-day visit here, including talks with Chancellor Helmut Schmidt. He came here from Brussels after signing a five-year trade pack with the European Common Market.

An agreement was also signed last month in Peking that allows the Chinese free experimental use ofr six-months of the West German-French Symphonie communications satellites, an agreement that Bonn ministers said was the first joint research program between China and the west.

Later this month, Deputy Prime Minister Ku Mu is scheduled to arrive her. He will be the highest ranking Peking official to visit Bonn.

Most West German officials believe Peking's relations with Bonn ane predominantly for practical purposes, since West German industrial knowhow and technology is highly regarded and the Germans have a good reputation for on-time delivery and service. The Chinese, like other countries, however, have also run into deficit problems, since they export to West Germany goods worth approximately half of what they import from Bonn.