The influences of the polyculture of Hyriopsis cumingii with fish in an open pond on the major parameters of water quality were analyzed.The eutrophication extent of the water body was analyzed according to the water quality indexes,such as pH and amounts of COD_(Cr),TN,TP,etc.The results showed that the quality of the water in experimental systems were good.In the mussel-fishes co-cultivating system,the content of COD_(Cr),TN and TP are reduced by 44.74%,70.81%and 70.45%respectively,by applying the co-culturing system in high temperature seasons.But the effect was not obvious during the seasons with low temperature.The results indicated that application of the fish-mussel co-culturing system are promising for improving the water quality,especially for the controlling of eutrophication.