Abstract: Rapid urbanization processes alter the regional underlying surface, leading to changes in the hydrological cycle. Such changes have made it difficult to explain the traditional water yielding and runoff routing mechanism, which necessitates our new exploration. In this paper, taking the Yangtze River Delta region as an example, we have conducted the experimental observations in basins with the different urbanization levels and different space scales. Via the collected first-hand hydrologic data, namely, rainfall, soil moisture, and streamflow with high resolutions (5 min time intervals), we adopted data-driven flood feature analysis to reveal the responses of the flood characteristics, such as flood lag time and peak discharge. Then, we discussed the response mechanism of rainstorm and flood in rapid urbanization areas. The main conclusions are as follows: ① Compared with other land use/cover types, the response of soil moisture for urban land are more intense under different rainstorm events (usually increasing more than 4% for the surface layer). The change of underlying surface in urbanized area affects the processes of surface runoff by influencing the dynamics of soil moisture. The soil moisture content of urban land and wasteland with low vegetation coverage showed steep rise and fall with the beginning and end of rainfall, while that of land use with higher coverage showed slow rise and decline trends. ② Flood peak lag time and peak discharge are mainly controlled by watershed scale. There have power law relationships between flood lag time and peak flow with watershed scale, and factors such as impervious surface and topography also have some influences on flood characteristics. ③ Rainstorm is a direct driving factor of flood in the Yangtze River Delta region. Rainfall characteristics are strongly correlated with the peak discharge, peak discharge per unit area, and runoff depth, with correlation coefficients above 0.49, 0.41 and 0.78, respectively. In general, the natural geographical characteristics and rainfall are the main factors affecting the flood characteristics, but the impact of the urban underlying surface on the flood characteristics cannot be ignored. The processes of rainstorm floods have been directed affected by the modification of soil moisture dynamics due to urbanization. Under the dual role of urbanization and climate change, the regional flood risk would be intensified in the future.