Abstract: The karst regions of Southwest China are prone to frequent rockfall hazards due to their complex geological structures and the combined effects of intense rainfall and active karst processes. This study investigates the failure mechanism and dynamic evolution of the Yiziyan rockfall that collapsed on June 20, 2023, in Bijie, Guizhou. Integrated UAV-derived point cloud data, field structural surveys, and a stochastic joint network, a 3D discrete-element model was developed to realistically represent the terrain and structural features of the slope. The model simulates the progressive destabilization process triggered by the coupled action of karst weathering-induced basal degradation and hydrostatic pressure within rear tension cracks. Numerical results reveal that basal erosion causes a progressive shift in the center of gravity, enhances crack connectivity, and intensifies stress concentration at the toe, ultimately triggering a typical toppling-type collapse. The simulated failure sequence —initiation, toppling, fragmentation, and deposition—is consistent with video records from the event, clarifying the spatial pattern of block breakage and debris accumulation. These findings provide new insights into the failure process of karst-induced rockfalls and offer a scientific basis for hazard assessment and risk mitigation in similar mountainous areas of Southwest China.