Abstract: Inorder to study synergic effect of hydrophyte (Pistia stratiotes) and compound microbial preparation purifying water in aquacultural ponds, an experiment, designed to have five treatments different in concentration of microbial preparation (1×10⁴, 3×10⁴, 5×10⁴, 7×10⁴ and 9×10⁴mL^-1 ,number of microorganism was measured by cfu), was carried out. Results demonstrate that the water purifying effect of the combination was significantly better than that of CK (with hydrophyte only); and the effect of removing total nitrogen(TN), total phosphorus(TP), COD, ammonia nitrogen (NH₄⁺-N), nitrite nitrogen (NO₂^--N) varied with the concentration of microbial preparation in the combination. Through synergic actions, the combination could not only significantly decrease TP,NH₄⁺-N and NO₂^--N contents in the eutrophied water, but also suppress increase in TN and COD contents. The findings indicate that the fortified hydrophyte-microorganisms system is an effective in-situ biological purification system for eutrophied aquiculture water. When hydrophytes in a pond reach 20% in coverage, it is proper to add 3×10⁴ mL^-1 of compound microbial preparation.