Urban floods caused by sudden heavy rainstorms are becoming more frequent and causing serious problems in many cities. Developing methods to simulate urban rainstorm floods is helpful in disaster prevention and mitigation. In this paper, we establish an urban pluvial flood simulation model based on diffusion wave approximation of shallow-water equations. The model takes full account of the characteristics in urban pluvial floods, and includes many improvements in simulation details. These details include building a consideration method, a rainfall consideration method, and so on. A new calculation method of water surface gradient is established, which is suitable for complex topology in urban pluvial flood simulation and can reduce unnecessary simulation error introduced by calculation methods. The accuracy and stability of the model are verified through simple cases with analytical solution and experiments with measured data. The results show that the new model is more accurate than common diffusion wave approximation models. A new treatment to avoid ‘checkerboard oscillation’ is established. In comparison with existing methods, the new method proved to be the best. A proof of concept shows that the new model can deal with complex situations and is helpful for urban drainage system planning.