A pond-based experiment was conducted from 26 April to 12 December 2012,in order to evaluate the effects of polyculture of freshwater mussel Hyriopsis cumingii in the ponds of silver carp Hypophthalmichthys molitrix and bighead carp Aristichthys nobilis,on water quality,plankton community and mussel growth performance.During the experiment period,four mussel densities were set up:fish monoculture pond(0/m³),low density mussel polyculture pond(0.8/m³),medium density mussel polyculture pond(1.0/m³)and high-density mussel polyculture pond(1.2/m³).The results revealed that water quality parameters(TP,PO₄-P,NH₃-N,NO₂-N and NO₃-N)of the ponds with mussels were significantly lower than the fish monoculture ponds.All water quality parameters of medium density mussel polyculture ponds were significantly lower than those of the other three ponds,except that its chemical oxygen demand(COD)and NH₃-N showed no significant difference with that of low density mussel polyculture ponds.The plankton densities in fish monoculture ponds were significantly higher than those of mussel polyculture pond.In mussel polyculture ponds,negative correlation was found between phytoplankton density and mussel culture density.Phytoplankton biomass of fish monoculture pond was significantly lower than that of mussel polyculture pond,while significantly lower than that of the low density museel polyculture pond.Phytoplankton biomass positively correlated with mussel density.The biomass of green algae and bare algae significantly increased in the polyculture mussel pond.the mussel survival rates were significantly higher in low density and medium density mussel polyculture ponds than that of high-density polyculture pond,but the values of mussel wet weight,shell length and shell width were the highest in the low density mussel polyculture pond.This study indicated that polyculture with freshwater mussel can effectively improve the water quality,control excess growth of harmful algae,accelerate the growth of chlorella and euglenophyta,increase the biomass of plankton,and improve survival and growth performance of the mussels.Finally,this study demonstrated that the optimal culture density of freshwater mussel in the ponds of silver and bighead carps was 1.0/m³.