Abstract: A fortified aquatic plants-microorganisms system in a Macrobrachium nipponense rearing pond was established by planting Hydrilla verticillata based aquatic plants and then adding a kind of compound microbial preparation,containing at least 10⁸ g^-1 viable bacteria,to study effect of the system on purifying the waterbody.Results show that after 40 days,by making full use of the enormous surface area of the auatic plants,the viable microbes grew and formed a fixed biofilm,thus increasing the number of bacteria and fungi in the plant area and its neighboring waters by 25%-69%,and intensifying the self-purifying effect of the waterbody.Through synergic actions,the fortified aquatic plants-microorganisms system could not noly improve utilization of nitrogen,phosphate and organic substances,thus promoting growth of the biological system per se,but also decrease NH₄⁺-N、NO₂^--N、NO₃^--N、TP and PO₄^3--P contents in the water,suppress rapid increase in TN and COD,increase dissolved oxygen(DO) and compete with alga for resources for growth,thus lowering abundance of alga in and stabilizing pH of the waterbody.The findings indicate that the fortified aquatic plants-microorganisms system is an effective in-situ biological purification system for Macrobrachium nipponense rearing water.The optimum aquatic plant coverage of the water surface is 40%,when the added compound microbial preparation is 1.0mg•L^-1 in biomass concentration.