[Objective] The evolution mechanism and catastrophic process of dam-break debris flows in the Bailong River basin were investigated in order to provide technical support for their early warning and prevention. [Methods] Ganjia gully in Hanwang Town， Longnan City was selected as the study area. Characteristics of source materials， topography， and rainfall were obtained through indoor remote sensing interpretation and field investigation. Combined with disaster-forming conditions such as regional geological structures and seismic activity， the movement characteristics of dam-break debris flows were systematically analyzed. Furthermore， three dam-break modes， including main gully， branch gully， and cascading dam-break， were simulated and comparatively analyzed using the numerical simulation platform OpenLISEM. [Results] ① The Ganjia gully watershed had abundant source materials and numerous collapse-slide masses， with rainstorms being the main triggering factor. These masses were prone to forming dam-break debris flows after blocking the channel. ② The evolution process of dam-break debris flows was divided into four stages： collapse-slide mass instability-channel blockage-landslide dam formation and breach-discharge amplification causing disaster. The catastrophic process involved the coupling of rapid conversion of gravitational potential energy into kinetic energy and the overall fluid impact pressure. ③ The simulation results showed that the main gully dam-break debris flow had a runout distance of 1 342.06 m， a deposition area of 1.38 km²， and a maximum deposition thickness of 21.64 m， slightly blocking the Bailong River. The branch gully dam-break debris flow was relatively smaller in scale， with a deposition area of 1.03 km² and a maximum deposition thickness of 15.37 m， causing no blockage to the Bailong River. The cascading dam-break debris flow was the largest in scale， with a runout distance of 1 365.64 m， a deposition area of 1.92 km²， and a maximum deposition thickness of 28.2 m， severely blocking the Bailong River and forming a barrier lake， which was highly likely to trigger an outburst flood and posed a serious threat to downstream residential areas. [Conclusions] ① Ganjia gully possesses unfavorable conditions， including steep terrain， abundant source materials， concentrated rainfall， and frequent seismic activity， making it a high-incidence area for dam-break debris flows. ② Dam-break debris flows generally undergo the blockage—impoundment—outburst stages， and their destructive power is significantly higher than that of general rainstorm-induced debris flows. ③ Different dam-break modes vary in scale， deposition range， and hazard severity， among which the cascading dam-break presents the greatest risk.