From the point of view for overland flow based on Saint-Venant equations, kinematic wave was introduced to describe the rainfall convergence process for urban surface, simultaneously，the linear superimposition was used when the theory for calculating the overland flow under the condition of even rain intensity was improved to calculate the flow process under uneven rain intensity, and the catchment was divided into several units in accordance with their properties, finally the flow process at the outlet was obtained by adding each u nit’s flow process. At last, the simulated results were verified using the measured data, and show a high degree of data fitting.