Abstract: On July 4, 2023, a high-elevation rock avalanche-debris flow occurred in Yitougou, Changtan Town, Wanzhou District, Chongqing, threatening the lives of 408 residents. To enhance the understanding of the dynamic mechanisms governing such high-altitude rock avalanches, this study conducted a numerical simulation of the event using the Smoothed Particle Hydrodynamics (SPH) method integrated with a Modified Johnson-Cook (MJC) constitutive model within the self-developed LPF3D simulation platform. The simulation results indicate that the motion process of the avalanche-debris flow can be divided into four distinct stages: (i) weakening of the slope due to external disturbances; (ii) frontal traction and initial acceleration; (iii) middle-rear compression and fragmentation; and (iv) energy dissipation and final deposition. Furthermore, the study reveals that the shear-induced fragmentation of the rock mass and its subsequent transformation into a debris flow are key mechanisms driving the high-speed and long-runout movement. This research not only deepens the theoretical insights into the dynamics of high-elevation rock avalanches but also provides quantitative support for hazard assessment and spatial prediction of similar events in mountainous regions.