Abstract: Under the combined pressures of climate change and rapid urbanization, flood risk in China’s coastal cities has intensified substantially, necessitating a systematic investigation of its spatial heterogeneity and underlying mechanisms at the national scale. Taking coastal cities in China as the research subjects, this study constructs a flood hazard evaluation index system. The CRITIC method is employed to determine the objective weights of the indicators, and the TOPSIS method is applied to comprehensively assess flood hazard levels for 54 coastal cities across China. Furthermore, the geographical detector model is used to characterize the spatial heterogeneity patterns and to identify their driving mechanisms. The results reveal a spatial pattern characterized by higher flood hazard levels in southern regions and lower levels in northern regions, with evident localized clustering. High-hazard cities are predominantly concentrated in highly urbanized regions, such as the Yangtze River Delta and the Pearl River Delta. River network density, the impervious surface ratio, and the proportion of coastal lowlands are identified as the dominant factors influencing the spatial heterogeneity of flood hazard. Although climatic factors exhibit limited explanatory power when considered individually, their interactions with topographic and underlying surface factors significantly enhance the spatial heterogeneity. These findings provide a scientific basis for differentiated flood prevention and disaster mitigation planning, as well as for climate-adaptive urban governance in coastal cities.