Abstract: Small watershed rainstorm-induced flash floods are characterized by sudden onset, high intensity, and unpredictability, often lacking comprehensive monitoring data, making it difficult to review and analyze the development and impact processes of such disasters. Based on a real-case study of the "20220809" extremely heavy rainstorm-induced flash flood disaster in a small basin in Southwest China, this research employs peak rainfall recurrence period analysis, small watershed design flood modeling, and numerical simulation techniques. Through a comprehensive analysis, the study systematically reconstructs the entire development and impact process of the disaster and validates the results using surveyed and measured data. The findings demonstrate that integrated multi-method analysis is highly effective for reviewing the entire process of small watershed mountain flood disasters.