Abstract: To solve the recurrent flooding of concave-down overpass bridges and the problem that storm water management model (SWMM) cannot simulate the water diffusion on the surface, a method of urban flooding simulation on concave-down overpass bridge by developing SWMM was presented in this paper. The method built high precision DEM of overpass bridge and obtained the topography of bridge. Then, the way of junction flooding that was modeled by SWMM was developed. It made the floods flow upon the topography of bridge. The method solved the problem that the junction flooding does not diffuse on the surface in traditional simulation. The modeling of urban flooding process on concave-down overpass bridge was realized. Results show that this method provides the support for the prevention and treatment of urban flooding on overpass bridges.