A rainwater outlet in the north moat of Dongcheng District of Beijing was selected for joint monitoring of water quality and quantity, and the characteristics of rainfall runoff pollution in the old city under light rain, moderate rain and heavy rain were identified respectively. The initial scour effect and sensitivity indexes affecting runoff pollution and their thresholds were analyzed, and the control scheme of source and end pollutants in the old city was formulated. The results showed that the pollutant concentration was higher in the early period of rainfall (20 min before light rain, 30 min before moderate rain, 10 min before heavy rain), and there was an early scour effect. The sensitivity of light rain to SS, TP and TN is higher than that of moderate rain and heavy rain, the sensitivity of moderate rain to COD and NH₃-N is higher than that of light rain and heavy rain, and the EMC concentration value of moderate rain and light rain is higher than that of heavy rain. For SS, NH₃-N and TP indexes, heavy rain, moderate rain and light rain had the largest variation range of pollutant concentration. When the runoff pollution reduction rate is 30%, the runoff volume of light rain, moderate rain and heavy rain should be controlled to be 25 m³, 35 m³ and 150 m³, or the rainfall in the early rainfall should be controlled to be 4 mm, 9 mm and 24 mm respectively.