Through the analysis of parameters and structure, it was found that the description of storm water management model(SWMM) on the process of rainfall-runoff formation in urban areas is consistent with urban runoff yield and concentration rule. The physical concept of SWMM is clear because the runoff yield analytical methods are based on hydrology, and concentration analytical methods are based on hydraulics. The parameters of SWMM almost have geometric or physical meanings, but there are some complementary(or dependency) relationships among some parameters. Therefore, it is necessary to reduce the influence of the equifinality for different parameters on parameter calibration. The composition of the process of rainfall-runoff formation in catchment area exit using the method of simultaneous summation shows that the interference between stormflows from different areas is neglected.