Abstract: Urban lakes are important surface water resources,which are of great significance for regulating runoff and maintaining ecological environment.Through the construction of water diversion project,the pollution of urban lakes can be effectively alleviated by improving the hydrodynamic and water quality conditions. In this paper,Balihu,an urban lake with serious water pollution in Jiujiang City,is taken as the research object. Aiming at the water pollution problem of urban lakes,the water quality status and main pollution sources of Balihu are found out through field water quality detection. The two-dimensional hydrodynamic water quality model of Balihu Lake based on DEM is established,and the parameters are calibrated and verified. Finally,based on the model,the improvement effect of the diversion project is analyzed,and the optimal diversion flow of Balihu is determined by using the cost and benefit evaluation method. The results show that the optimal diversion flow of Balihu is 70 m3/s. The corresponding working condition is working condition 4（Xinkai River diversion 50 m3/s,Saicheng Lake diversion 20 m3/s）. The net benefit is 589 800 yuan/time when it runs for 8 days.