Abstract: Large high-altitude landslides are widespread along the Deqin reach of the upper Lancang River, exhibiting diverse types, pronounced deformation, and complex evolutionary pathways that readily cascade into basin-scale hazard chains threatening towns and infrastructure. However, the predisposing settings, developmental patterns, and geomechanical models for this reach remain insufficiently constrained. Here we implement InSAR, UAV, field investigation, borehole drilling, and geophysical surveying to delineate the spatial distribution and structural architecture of large high-altitude landslides and to resolve the formation and evolution mechanisms of representative cases. Results show that, under the combined control of extreme relief and lithology - structure coupling, the Foshan township -Yanmen township reach is dominated by large high-altitude landslides, whereas the Yezhi township -Yanmen township reach is characterized by paleo-landslide deposits; toppling deformation zones are pervasive across the study area. We identify lithology and its assemblage as key factors governing landslide development, and demonstrate that endogenic - exogenic coupling drives formation and evolution: crustal uplift and active faulting provide the tectonic background; deep fluvial incision and long-term unloading promote rock mass toppling and the formation of high-altitude tensile headscarps; rainfall and groundwater processes elevate pore pressure and accelerate internal weakening. On this basis, we synthesize three classes and eight types of geomechanical failure modes, with toppling dominated failure as the principal mode. Further analyses indicate that landslide deposits are prone to reactivation under earthquakes, intense rainfall, and engineering disturbances. The findings clarify the causative mechanisms and evolutionary pathways of large high-altitude landslides in the Deqin reach and provide a scientific basis for hazard chain mitigation and siting of major engineering works.