Aiming at the serious problem of confluence overflow pollution in Beijing sub-center section of the North Canal, this paper calculated the river pollution carrying capacity of the overflow event by means of monitoring and simulation and selected the non-deterioration of the water quality of the receiving water body in the outlet cross section as the management goal. The frequency of exceeding standard was taken as the CSO control index and the results were compared with those taking the overflow frequency as the CSO control index. The results show that the average annual overflow frequency of each outlet from 2013 to 2017 is 7 to 9, and the average overflow volume is 0.142.7 million m3. There were 19 rains, of which 10 overflowed in 2017. All overflow events exceeded the receiving capacity of the river water body. Taking the overflow frequency and exceeding standard frequency as CSO control indexes, the input-output ratio was the best when the valued of frequency was set as 3 times. The corresponding regulation and storage volumes were 18.116 m3 and 13.899 m3 respectively, the control rate of annual overflow volume can reach 52% and 44%, and the load control rate of overflow pollution can reach 59% and 51%. Using the exceeding standard frequency as the control index can make full use of the water environment capacity, reduce the scale of CSO treatment project, and enrich the engineering measures.