[Objective] This study aims to reveal the evolutionary mechanisms and catastrophic processes of dam-break type debris flows in the Bailong River Basin. [Methods] Taking Ganjia Gully in Hanwang Town, Longnan City as the study area, source materials, topography, and rainfall characteristics were obtained through indoor remote sensing interpretation and field investigations. Combined with regional geological structures, seismic activity, and other hazard-inducing conditions, the movement characteristics of dam-break type debris flows were systematically analyzed. Furthermore, the numerical simulation platform OpenLISEM was employed to simulate and compare three dam-break scenarios: main gully, tributary gully, and cascading dam-break debris flows. [Results] The results indicate that: (1) The Ganjia Gully basin is rich in source materials and contains numerous landslides. Intense rainfall is the primary triggering factor, and once landslides block the channel, dam-break debris flows are highly likely to occur. (2) The evolutionary process of the cascade outburst debris flow can be divided into four stages: landslide failure – channel blockage – formation and breaching of the landslide dam – discharge amplification and disaster formation. The catastrophic process is characterized by the coupled action of the rapid conversion of gravitational potential energy into kinetic energy and the overall impact force of the debris flow. (3) Simulation results show that the main-gully dam-break debris flow traveled 1342.06 m, with a deposition area of 1.38 km2 and a maximum thickness of 21.64 m, causing slight blockage of the Bailong River. The tributary-gully dam-break debris flow was relatively smaller in scale, with a deposition area of 1.03 km2 and a maximum thickness of 15.37 m, without blocking the Bailong River. The cascading dam-break debris flow was the largest, traveling 1365.64 m, with a deposition area of 1.92 km2 and a maximum thickness of 28.2 m, severely blocking the Bailong River and forming a landslide-dammed lake, which poses a high risk of dam-break flooding and a serious threat to downstream settlements. [Conclusions] (1) Ganjia Gully is highly susceptible to dam-break debris flows due to its steep terrain, abundant source materials, concentrated rainfall, and frequent seismic activity. (2) Dam-break debris flows generally undergo blockage–impoundment–outburst stages, with destructive power significantly greater than that of typical rainfall-induced debris flows. (3) Different dam-break scenarios vary in scale, deposition range, and hazard level, among which cascading dam-break debris flows present the greatest risk.