Due to their special landforms, urban hilly forest parks influenced by rain-flood in city have large impact on urban safety. Taking an example of Nanjing Purple Mountain National Forest Park, based on DEM and NDVI, factors cause rain-flood of Nanjing Purple Mountain National Forest Park were analyzed. Moreover, surface runoff areas were simulated. Then we analyzed the causes of the areas affected by rain and flood from the source and route levels. The results could be used in practice, which indicate that the runoff channel and high storm flood risk area simulated by data model constructed by DEM and NDVI are consistent with reality. Aim to give full play to the function of rain flood controlled by Nanjing Purple Mountain National Forest Park and guarantee the safety of urban rain-flood, a reasonable whole process of rain-flood management strategy is proposed for the three aspects of source, approach and terminal.