Objective The Batang fault zone located in the eastern Qinghai–Xizang Plateau exhibits intense tectonic activity and frequent seismic events, resulting in the extensive development of large ancient landslides that constantly threaten the safety of highways, railways, rivers, and villages. However, existing studies lack clear understanding of the reactivation mechanisms of large ancient landslides and insufficient in−depth analysis of influencing factors, constraining effective early warning and prevention of landslide hazards.

Methods This study takes the Bogexi ancient landslide in the eastern Qinghai–Xizang Plateau as a case study, combining Sentinel−1 satellite SBAS−InSAR deformation data (2017—2025), long−term precipitation records, and geological investigations to analyze its deformation mechanisms and influencing factors.

Results The Bogexi ancient landslide exhibits a “long tongue−shaped” planform, with a longitudinal length of 2.5 km, an average width of 800 m, and a volume of approximately 208×10⁵ m³, presenting characteristics of “tectonically segmented sliding control”. InSAR surface deformation monitoring reveals spatial heterogeneity in landslide deformation, with the backwall of landslide being relatively unstable, showing an average annual deformation rate of approximately −48.5 mm/a. The extremely strong deformation zone in the middle part of the landslide exhibits a maximum deformation rate of −267.15 mm/a, while the extremely strong deformation zone at the landslide toe shows a maximum deformation rate of −177.11 mm/a, displaying multi−stage evolutionary characteristics. Based on time−series analysis, the evolutionary process of the Bogexi ancient landslide is divided into three stages: initial creeping (2017—2020), heavy rainfall−triggered accelerated sliding (2020—2021), and uniform deformation (2021 to present), exhibiting response characteristics of “rainfall−stage triggering” and “precipitation−controlled deformation lag time”, with a maximum cumulative deformation of −3358.62 mm. The deformation rate profile and field geological investigation reveal that the bedrock resistance zone of the landslide temporarily constrains the through−going sliding between the upper and lower sliding masses, presenting a compound deformation mechanism coupling thrust−type movement in the middle−rear section with traction−type movement at the frontal zone.

Conclusions The composite deformation mechanism reflects the control mechanism of fault zone activity and heavy rainfall coupling on landslide deformation. This study deepens the understanding of ancient landslide reactivation mechanisms under tectonic−rainfall coupling control and provides scientific reference for landslide monitoring and early warning in high mountain−canyon regions.