Abstract: This paper deals with the application of man-made water weeds in freshwater fish-farming and its biological mechanism. Putting the man-made water weeds in mixed-fish-farming pond can enlarge the superficial area, accerelate organic substances mineralization rate, enrich the nutrient elements in water body, and increase the amounts of algae and heterotrophic-bacteria. It is found that there are more zooplankton in test-pond than in comparative-pond. Since the bait is increased, the fish yield increased by about 15. 6%.