Abstract: Located in the transition zone between mountains and plains, piedmont towns face the dual pressures of rapid upstream runoff and sluggish downstream drainage, making them highly vulnerable to flood disasters. This study focuses on the Sanjiadian Historical and Cultural District in Mentougou, Beijing. Using the HEC-RAS two-dimensional hydrodynamic model, it simulates the flood evolution and inundation processes under the ‘23·7' extreme basin-wide rainstorm scenario. The study reveals a coupled hazard mechanism characterized by ‘rapid flood convergence, drainage congestion due to backwater effects, and superimposed internal flooding' in piedmont towns. It proposes a flood response framework across three dimensions: flood retention, flood prevention, and flood response. The results show that the flood risk in piedmont towns stems essentially from the coupling of three factors: rapid upstream convergence, orographic enhancement of rainfall, and limited drainage capacity in the downstream plain. Through integrated measures-including the restoration of flood storage/detention areas, the reopening of flood discharge corridors, the rehabilitation of critical hydraulic nodes, and phased operational management-the flood peak can be effectively attenuated, the recession time shortened, and the inundation extent significantly limited. This research provides a theoretical basis and technical reference to enhance the flood resilience of piedmont towns in the context of mountain flood disasters.