In view of water ecological problems such as insufficient hydrodynamic power and deteriorating water quality in Xinghai Lake, a two-dimensional hydrodynamic water quality model was established with chemical oxygen demand (COD) as the water quality indicator. 12 working conditions under different hydrological characteristics and water diversion volumes were calculated to analyze the changes and spatial distribution of hydrodynamics and
COD in the lake area under different water quantities from the Yellow River. The results show that the diversion of the Yellow River water can effectively improve the hydrodynamics and water quality of Xinghai Lake, reduce the area of stagnation and COD concentration. Under the same hydrological working condition, COD concentration gradually decreases with the increase of water diversion and its reduction rate gradually increases; under the same water diversion conditions, COD reduction rate in high flow years is generally higher than that in low flow years and normal flow years. In addition, water diversion volumes of 14×106 and 7×106 m3 are needed to meet the Class Ⅲ water standard of surface water in low flow years and normal flow years respectively.