Abstract: In order to explore the characteristics of initial rainwater pollution in hilly towns in the Yangtze River Basin and the interception effect of initial rainwater storage tanks, this paper took a part of the diversion system catchment area in Yongchuan District, Chongqing City as the research object, and the concentrations of COD, SS, TN, TP and NH₃-N were monitored at the stormwater outfalls. The changes of each pollutant with the time of rainfall and the effect of rainfall characteristics were analyzed. Meanwhile, the first flush effect of rainfall on the catchment area was judged by using the dimensionless cumulative curves and the b-parameter method. The SWMM pipe network model was used to simulate the stormwater outfall loads and interception and storage effects in the catchment area under different recurrence periods. The results showed that each pollutant in the drainage area showed the initial flush effect under different rainfall conditions, and the intensity was weak in general. The concentrations of most pollutants showed an overall decreasing trend during the rainfall and fluctuated little in the late rainfall period. There was a certain correlation between each rainfall characteristic factor and the average concentration of each pollutant in the rainwater outfall. For the initial rainwater storage pond, when the interception ratio was less than 30%, the growth rate of the pollutant reduction rate was large, and when the interception ratio was greater than 30%, the growth rate of the pollutant reduction rate slowed down. When the recurrence period increased, the reduction rates of SS and COD increased more, while the reduction rates of TN, TP and NH₃-N increased less. Under the simulation of the recurrence period of 0.5 a and 1 a, when 20% of the rainwater runoff volume was intercepted the maximum pollution load could be controlled by 54.71%.