In view of the insufficiency of the basis for evaluating the drainage capacity of pipe networks in urban waterlogging control construction, based on the GAST-SWMM two-dimensional hydrodynamic coupling model, taking Xi’an Fengxi New City as the research area, the proportion and flow variation laws of pipe network and surface runoff drainage under different cumulative rainfall conditions were simulated and analyzed to quantitatively analyze the changes in pipe network drainage capacity. The results show that the emission ratio of pipe network to surface runoff is significantly different under different cumulative precipitation. With the increase of cumulative precipitation, the ratio increases first and then decreases. When the cumulative precipitation is 10 mm, the minimum value is 0.543, and the maximum value is 2.241 when the cumulative precipitation is 80 mm, and then it shows a downward trend.The emission ratio of pipe network and surface runoff is not sensitive to the rain peak coefficient. The maximum difference rate is 12.96% when the cumulative precipitation is small, and the maximum difference rate is only 3.98% when the cumulative precipitation is large. The increase of the rain peak coefficient will increase the peak flow of each part of the drainage. When the cumulative precipitation is 100 mm, the maximum flow of pipe network drainage is 6.26,7.45, and 9.40 m³/s, respectively when the rain peak coefficient is 0.2,0.5, and 0.8, and the maximum flow of surface runoff drainage is 3.19,3.90, and 4.84 m³/s, respectively.