An enclosure experiment was conducted to evaluate the effect of co-culturing freshwater pearl mussel Hyriopsis cumingii in a fish polycultural system consisting of grass carp Ctenopharyngodon idellus,gibel carp Carassius auratus,silver carp Hypophthalmichthys molitrix and bighead carp Aristichthys nobilis on fish yield and water quality.Two treatments were examined.In treatment Ⅰ,polyculture of grass carp,gibel carp,silver carp and bighead carp was used.In treatment Ⅱ,the mussel was co-cultured in polycultural system of grass carp,gibel carp,silver carp and bighead carp used in treatment Ⅰ with a mussel-fish ratio of 1：1.The experiment lasted 78 days,during which water samples were collected regularly,and species composition and biomass of phytoplankton,primary productivity(P),community respiration(R),dissolved oxygen(DO),pH,sechii depth(SD),main ions(CO₃^2-,HCO₃^-,Cl^-,SO₄^2-,Ca²⁺,Mg²⁺,Na⁺+K⁺),total alkalinity,total hardness,ammonia(TAN),nitrite,nitrate,reactive phosphate,total nitrogen(TN),total phosphorus(TP),total organic carbon(TOC),chemical oxygen demand(COD_Mn),biological oxygen demand(BOD₅)were monitored.The results showed that co-culturing H.cumingii significantly decreased the Ca²⁺ concentration in the enclosures,but did not significantly affect the other water quality parameters,such as species composition and biomass of phytoplankton,diversity of phytoplankton(Shannon-Weaver's index,Margalef's index,Pielou's index),P,R,DO,pH,SD,CO₃^2-,HCO₃^-,Cl^-,SO₄^2-,Ca²⁺,Mg²⁺,Na⁺+K⁺,total alkalinity,total hardness,TAN,nitrite,nitrate,reactive phosphate,TN,TP,TOC,COD_Mn and BOD₅.Yields of grass carp,gibel carp and silver carp were relatively high,while production of bighead carp and accumulation of TN,TP,COD_Mn,BOD₅ and TOC were low in treatment Ⅱ,compared with treatment Ⅰ.Therefore,co-culturing the mussel in a fish polycultural system of grass carp,gibel carp,silver carp and bighead carp not only enhanced economic income of aquaculture practice,but also reduced the accumulation of nitrogen,phosphorus and organic matter.