The study of landslide initiation mechanisms and dynamic process analysis is important for monitoring, predicting, and controlling landslide occurrence. A giant landslide that occurred on July 19, 2018, in Yanyuan County, Southwest China, was investigated, providing a research example. In this paper, the failure mechanism and dynamic process of the Boli landslide were analyzed using field investigations, laboratory experiments, unmanned aerial vehicle (UAV) photogrammetry, remote sensing images, hydrological calculations, and numerical simulations. The results indicated that the antecedent rainfall was not the direct factor to saturate the sliding mass. The runoff supply provided by the comb-like channel group upstream of the landslide reached 709.5 mm, which was an important hydrodynamic triggering point for accelerating the sliding mass to saturation and instability. Numerical simulation results showed that the evolution of the landslide included three stages: low-speed initiation, accelerating sliding, and flow deposition. The maximum sliding speed of the landslide occurred during the second stage and reached 39 m/s. From the source zone to the Taozi Gully, the silt content decreased from 67.9 to 40%, which was an internal particle–triggering factor of the alteration of the sliding mass from the liquefaction-resistant state to the liquefaction-prone state. The study has revealed the failure mechanism of the landslide and presented its dynamic process, which could be used to provide a reference for the study of similar landslides mainly induced by runoff supplied by such channel groups.