To simulate large scale urban drainage process with high-efficiency and high-precision, and master the mechanism of urban surface runoff and pipe drainage pattern, taking the 310km² urban area in Yinchuan as the studied area, a GAST model with coupled surface runoff and urban underground pipe network drainage process, combined with characteristics of local wastewater treatment plants, pipe networks, topography, land use, etc., is applied to raise an efficient simulation method for rapid modeling of urban drainage processes based on complex high-density pipe network structure optimization technology, which is based on technologies including universal DEM data processing, land use type classification, combined drainage network and wastewater treatment plant generalization, etc. The accuracy and efficiency of this modeling method is verified with both ideal terrain and actual terrain. The results show that:applying deletion treatment on the primary branch pipe has insignificant influence on the simulation results of the actual pipe network drainage process. In the 20.46km² area, the number of 1D-2D coupling nodes is reduced by 27.8%, and the calculation efficiency of the model can be improved by 12.18%. The larger the study area is, the more significant improvement in calculation efficiency there would be. The errors between simulation results and measured values are smaller than 6%, which indicates that the proposed method can greatly improve the calculation efficiency while the simulation accuracy ensured.