In order to reveal the temporal and spatial evolution law of precipitation during flood season in the core city group of the Yangtze River delta under the background of urbanization, based on the hourly precipitation data of 42 rainfall stations in the study area from 1978 to 2021, the temporal and spatial variation characteristics of precipitation in the flood season and the impact law of urbanization on precipitation were explored. The results show that, in terms of spatial distribution, the threshold value of extreme precipitation in the flood season gradually decreases from north to south; in terms of time scale, the precipitation in the study area in the flood season generally shows a significant increase trend, and the precipitation changes in the post-flood season dominate the precipitation changes in the entire flood season; the increase rate of precipitation in urban areas is basically higher than that in suburban areas, and the growth rate of total precipitation during the flood season in urban areas and suburban areas of Jiangsu, Zhejiang, and Shanghai shows a decreasing trend; Urbanization has a more significant impact on the rainfall variation between urban and suburban areas in Shanghai. The impact degree on the difference between urban and suburban areas of different precipitation indicators is the largest for the extreme precipitation in the post-flood season, followed by the extreme precipitation in the flood season, and the smallest for total precipitation in the flood season.