To fit the irregular boundaries of the Wuli Lake, quadrilateral mesh fitting has been used and a two-dimensional flux vector splitting(FVS)scheme flow-pollutant coupled model has been established, which was verificated after then. By using this model, the dynamic process of the impact of designed storm runoff on the water quality of Wuli Lake was simulated, the COD_Mn, NH₃-N, TP and TN exchange fluxes at Baojie bridge section were analyzed. The result showed that in the designed typical storm conditions, the Maligang River got large flow of water which had a great impact on the water quality of West Wuli Lake. After 3-day designed storm occurred, runoff sewage began to affect the West Wuli Lake, and the concentration reached the peak after the fourth day, which changed the water quality from class Ⅲ to class Ⅳin some lake district, but the duration is short, which then decreases rapidly.