The SWMM model is the most widely used urban storm water management model in the world. It is of great significance to study the simulation capabilities of the SWMM model. Based on a brief introduction to the SWMM model, its simulation ability to deal with the rivers in the urban area and the free-surface-pressure flow was studied and analyzed. The problems existing in processing and generalizing urban rivers when using SWMM model were analyzed by comparing the results of the SWMM model with a common river network model, and the feasibility of using the SWMM model to deal with urban river channels was verified. The classical circular drainage network with complex flow patterns was used to study and analyze the SWMM model's ability to deal with the full flow and the pipeline generalization strategy. It is proved that the SWMM model can be applied to the calculation of the water flow in urban river with proper section spacing, there is no significant differences from the results of the general river network model, and the results show that SWMM has the ability to simulate the free surface-pressurized flow. In the presence of very long pipes, to add intermediate nodes properly can improve the simulation results to a certain degree.