Storm Water Management Model (SWMM) is a widely used urban rainwater simulation and analysis software, the sub-catchment is the basic calculation unit in the SWMM model. Through the analysis of the rainfall runoff hydrological conceptual model, runoff yield and concentration mathematical model for SWMM sub-catchment, some suggestions on parameter assignment are put forward the application. In order to solve the problem of sub-catchment width value assignment，this paper proposes to take the length of the overland flow path as the basic method to calculate the width of the sub-catchment area, and to calculate the length by weighting the length of sub-catchment concentration.