Abstract: Abstract：[Objective] The Yahuokou landslide in Zhugqu County, Gansu Province, represents a large-scale creeping landslide in the Bailong River Basin. A major reactivation event in 2019 caused substantial economic losses. However, the mechanisms governing its instability remain unclear, limiting effective risk mitigation.[Methods] This study combines UAV photogrammetry, field surveys, ring shear tests, and numerical simulations to investigate the landslide’s deformation patterns, moisture-dependent strength weakening of sliding zone soil, and instability mechanisms. [Results] (1) The landslide is currently in a creeping state, with severe rear-edge deformation exposing sliding zone soil composed of weathered carbonaceous slate. This material contains approximately 26.4% clay and ~52% hydrophilic clay minerals, indicating high water sensitivity. (2) Under long-distance shearing, the sliding zone soil exhibits pronounced strain-softening behavior, with a maximum softening ratio of 57.2%. An increase in water content from 9% to 18% results in a 45.49% strength reduction and a 0.24 decrease in the root-mean-square roughness (Z?) of the shear surface. SEM images reveal face-to-face alignment of clay minerals, reflecting oriented microstructural rearrangement. (3) Numerical simulations indicate that the landslide remains generally stable under natural conditions but undergoes significant deformation under saturated conditions. Slope models with through-going fractures experience much faster infiltration and saturation of the sliding zone soil compared to intact slopes. [Conclusion] The landslide exhibits a characteristic “tensile-front and thrust-rear” deformation-failure pattern. The strong water sensitivity of weathered carbonaceous slate in the sliding zone is the dominant internal control on slope stability. Seasonal concentrated rainfall serves as the primary external trigger, while the presence of continuous fractures substantially accelerates infiltration and strength reduction in the sliding zone. These findings provide insights into the instability mechanisms of similar creeping landslides and offer valuable references for hazard assessment and mitigation in comparable settings worldwide.