On October 11, 2018, Baige landslide occurred in the upper reaches of the Jinsha River, and this was the most influential landslide event in Tibet since the Yigong landslide in 2000. The formation conditions, stability, monitoring and early warning of the landslide have been studied and reported by many experts, but the geo-mechanism of landslide has seldom been studied. On the basis of regional geological analysis, field survey, mapping and comprehensive research, the authors put forward the failure mechanism of the bank slope of the mélange in the Jinsha River structural suture zone under the sustained gravity, which mainly considers such factors as the structure of slope rock mass controlled by fracture, the weakening of the structural plane by the action of water and weak altered interlayer, and the scale and failure mode of the landslide controlled by unloading. The results have certain guiding significance for early recognition of potential landslide and risk prevention and control along the Jinsha River.