Taking the rainwater pipe network system of Heping District in Shenyang as an example, the Chicago rain type is selected as the design rain type, and the SWMM is used to simulate the drainage process of the storm water pipe network system under different heavy rainfall recurrence periods and out-river design flood levels. The results show that with the increase of rainfall recurrence period, the drainage capacity of rainwater pipe network increases, but the degree of waterlogging increases. The effective drainage coefficient of rainwater pipe network system decreases with the increase of rainfall recurrence period and the degree of change slowed down at high recurrence period. There is a significant positive correlation between the total drainage capacity of the network system and the difference between the river water level and the average ground elevation of manholes and the correlation coefficient R is 0.992, and it shows an approximate linear relationship. Pumps are essential in districts where submerged discharge occurs potentially. In this case, the pump enhances the drainage capacity of the pipe network system by 36.3%, while for the free flow area, the pumping station may not contribute to the drainage capacity of the system basically.