Abstract: To solve the problem of SWMM (storm water management model) that flooding process cannot be simulated on city surface, a method of urban flooding simulation by developing SWMM was presented in this paper. The DEM (digital elevation model) data of city as the storage in the model was generalized and the model was operated through modifying the model construction. And the flooding water flowed through the topography of storage bottom. The flooding process on the surface was calculated depend on the model result and DEM data. The method was verified by a case study. Results prove that SWMM can be used to simulate the flooding depth and flooding area on the surface. The urban flooding simulation was achieved. A reference for the research of urban flooding prevention was provided.