[Objective] The characteristics and causal mechanisms of low-frequency debris flows in the Qinling Mountains was analyzed in order to provide a scientific reference for their prevention and mitigation. [Methods] The “8·11” Jiwozi flash flood and debris flow of 2023 served as the research subject. Using field investigations and numerical simulations, actual precipitation frequencies were used to reverse engineer the formation process of the flash flood and debris flow, and a risk assessment was carried out. [Results] ① Intense rainfall accumulated quickly, forming torrents that erode loose materials in upstream channels. This process, known as the “fire hose” effect, triggered mountain floods and debris flows. Assisted by steep terrain, these flows eroded channels across the watershed, and a significant amount of sediment spreads downstream, forming alluvial fans and blocking river channels. ② Simulations under actual rainfall frequencies showed that the sediment deposition depths across the watershed ranges from 0.04 to 5.63 meters, with a maximum flow velocity of 7.43 m/s. The area of the alluvial fan was 1.91×10⁴ m², and the one-time discharge volume was 78 271 m³. ③ Based on flow velocity, sediment depth, and hazard range, three danger zones were identified (high, medium, and low). The area on the western side of the alluvial fan, with higher elevation, was classified as a low-risk zone, accounting for approximately 10% of the fan area. The central area and the area from the edge of the fan to the road were classified as medium-risk zones, comprising 62% of the total area. The southeastern and southwestern areas of the alluvial fan were high-risk zones, accounting for 28% of the area. [Conclusion] The “8·11” Jiwozi flash flood and debris flow in 2023 was a typical low-frequency flash flood and debris flow disaster triggered by extreme short-duration heavy rainfall. The FLO-2D model effectively simulates the movement and deposition processes of the debris flow and determines the hazardous areas.