Abstract: Frequent flash floods occur in small mountainous watersheds of China due to extreme weather.On July 22,2023,a short-duration extreme rainfall-induced flash flood struck Fuyang District,Hangzhou,Zhejiang Province,causing severe economic losses and casualties.This study focuses on the Dayuanxi basin,using the HEC-HMS hydrological model with the SCS (Soil Conservation Service) curve number loss method and SCS unit hydrograph to simulate runoff generation and confluence processes.The Muskingum-Cunge method was applied for channel routing,and flow hydrographs were extracted at the watershed outlet and key cross-sections.Post-disaster surveys identified 17 flood marks and four major cross-sections,supporting an in-depth analysis of flood response and disaster mechanisms.Results show that the simulated peak discharge at the watershed outlet was 169.5m³/s,with a relative error of 0.02% between the simulated peak water level and observed flood marks.Errors at the other cross-sections did not exceed 1.1%,confirming model reliability.The primary trigger factor was short-duration intense rainfall,with a three-hour maximum precipitation of 198.1mm,exceeding a 100-year return period.The watershed’s unique topography facilitated rapid flood convergence,causing a sudden water level rise in densely populated areas,a key factor in the disaster.Additionally,human activities further exacerbated flood impacts.Strengthening rainfall and hydrological monitoring,improving flood control planning,and managing human activities,especially in flood-prone areas,are key to reducing flash flood losses.These findings provide valuable insights for regions prone to similar flash flood hazards.