Aiming at the problem of serious overflow pollution and black odor of urban water body in rainy days, taking the combined pipe network system in a high-density urban area of Yinchuan City as an example, based on SWMM model, under the conditions of short-term design rainfall and long-term design rainfall, the impact of “grey-green” synergistic measures, such as combined sewer overflow (CSO) storage tank, mixed and wrong connection of rainwater and sewage pipelines and sponge transformation of green belt, on CSO pollution were simulated and analysed. The results show that under the short-term and long-term design rainfall conditions, four schemes of CSO storage tank, mixed and wrong connection of rainwater and sewage pipelines, sponge transformation of green belt, and “gray-green” combination measure, can lead to the increase of the amount of overflow water and the load of overflow pollutants with the increase of precipitation, and the overflow reduction rate decreases gradually. Among them, the “gray-green” combination measure has the most significant effect on the reduction of overflow pollution. When the return period is less than 5a, the reduction rate of overflow water volume and the reduction rate of overflow pollutant load basically reach 80%. When the rainfall condition is moderate rain, the pollutant load reduction rate basically reaches 75%. When the return period is 20a, the reduction rates of overflow water and the load reduction rates of TSS, COD, TP and NH4+-N reach 64%, 70%, 70% and 70%, respectively. When the rainfall condition is heavy rain, the reduction rates of overflow water and the load reduction rates of TSS, COD, TP and NH4+-N reach 28%, 32%, 26%, 31% and 33%, respectively.