In order to resolve the singleness of reaction principle of pollution factor in the 2-D water quality model, the basic equation of WASP5 water quality model (circle of total nitrogen) was taken as a source item, and a 2-D unsteady water quality and water quantity numerical coupling model was established by finite volume method (FVM) and flux vector splitting (FVS). The coupling model was applied to simulate the water quality and water quantity in a certain lake. The simulation result shows that the coupling model is reliable and practical.