Catchment area is not only the structural unit of runoff control, but also an important input data in hydrological model. In urban areas, the results obtained using the traditional catchment division method are quite different from the actual situation due to the complex underlying surface characteristics. In this paper, a new catchment division method is proposed based on the existing watershed division methods and by combining with the actual runoff characteristics of urban areas. First, the elevation information of different objects affecting the surface confluence path was integrated into DEM. Then the buffer analysis of the generalized drainage pipe network was carried out and fused into the DEM data in the corresponding range, and the DEM was further optimized. We determined the appropriate DEM spatial resolution to ensure the rationality of the division of sub-catchment area based on the study area ranges. Finally, the traditional D8 algorithm was used to draw the molecular catchment area according to the refined DEM data. Taking Haining area as the study area, and comparing the new method with three traditional methods, the experimental results showed that the refined DEM reflects the surface elevation more truly, and the watershed division results can accurately depict the surface runoff process and improve the rationality of the division of catchment area.