The relationship between extreme rainfall and maximum runoff in Xuyi County, Huai’an City, Jiangsu Province, China was studied by using the Chicago rain pattern method, statistical analysis, and literature review. From 1957 to 2020, the average precipitation was 1015 mm with a fluctuating upward trend mainly in July-August which accounted for 40% of the total annual precipitation. The maximum evaporation occurred in June, accounting for 12.7% of the total annual evaporation. From 1957 to 2020, the average maximum daily precipitation was 45 mm, while the maximum precipitation was found as 233.2 mm on July 27, 1970. In the 120-minute rain pattern, the maximum precipitation coefficient was 0.4, and the rainfall intensity showed an unimodal distribution. The total annual runoff control rate increased from 60 to 95%, the corresponding design precipitation increased from 16.3 to 71.5 mm, and the growth rate of design precipitation became faster.