Abstract: In October and November 2018, two large-scale high-altitude long-runout landslides successively occurred in the central section of the Jinsha River tectonic mélange belt, triggering a cascades disaster chain of landslide-dammed lake. The source areas of these landslides were located at Baige Village, Jiangda County, Changdu City, Tibet Autonomous Region. This study focuses on the long runout movement characteristics of the Baige landslide. We investigated the topographic features of the landslide area through unmanned aerial vehicle (UAV) surveying. Laboratory microscopic characterization was used to analyze the lithological properties of the mélange rock. The ambient noise dispersion measurements were used to explore the spatial structure of the mélange rock mass in the source area, combined with the high-speed undrained ring shear test to analyze the dynamics of the sliding zone material. This study reveals that: 1) The mélange rocks of the source area are primarily composed of chloritized metamorphic siltstone and illitized metamorphic slate. These rocks contain a high proportion of clay minerals, which are prone to weathering when exposed to water, leading to a reduction in strength. 2) The spatial distribution of blocks within the mélange rock mass significantly affects the formation and shape of sliding zones. Sliding zones tend to form along zones of weakness in both the blocks and the matrix, exhibiting a failure mode characterized by block-bypassing mechanisms. 3) The saturated sliding zone sample generates high pore-water pressure during undrained rapid shear process, resulting in significant strength degradation. Its peak shear strength and residual strength are 67% and 60% of that under dry conditions, respectively. The results demonstrate that the combination of the spatial structure and strength deterioration characteristics of mélanges is a key factor causing the frequency of high-altitude long runout landslides in this region. The shear strength characteristics of mélanges controlled the long-runout movement of the landslide. This research provides a foundation for the subsequent potential deformation and failure of the Baige landslide slope. It also offers valuable insights for slope stability analysis and landslide disaster prevention in tectonic mélange areas.