Lake Dong located in Wuhan is the largest urban lake in China. It has lost ecosystem functions with the economic development in recent years. The Lake Dong Ecological Water Network Project will reestablish the hydraulic connection between the Yangtze River and Lake Dong by diverting water from the river. In order to make a scientific water diversion scheme, a hydrodynamic and water quality model was established. Orthogonal curvilinear coordinate in horizontal direction and sigma coordination transformation in vertical direction were used. The model was verified using the observed COD concentration and applied to simulate three scenes of water diversion. By comparing the average COD of some sub-lakes in different scenes, a tactics of water diversion was proposed so as to provide a scientific basis for the project.