Abstract: The sliding zone soil,a critical component controlling landslide stability,exhibits shear strength that is highly dependent on water content. Although previous studies have extensively explored the macroscopic relationship between water content and mechanical properties,the micro-mechanisms governing moisture-induced shear strength deterioration remain unclear. The purpose of this study is to unravel the microscale interactions between water content variations and shear strength reduction in sliding zone soils. Methods including X-ray diffraction(XRD),large-scale triaxial tests,the filter paper method,and scanning electron microscopy(SEM)were employed to systematically analyze mineral composition,matric suction,microstructure,and shear strength under water contents ranging from natural(11%)to saturated(19%)conditions. The results demonstrated that: (1)matric suction decreased with increasing water content,and a critical suction threshold triggered a transition from a nonlinear to an approximately linear suction-strength correlation,accompanied by a sudden drop in shear strength; (2)multi-scale microstructural deterioration mechanisms revealed by SEM analysis showed that the synergistic effects of intergranular contact weakening and pore connectivity evolution induced a secondary abrupt shear strength reduction in sliding zone soils. In conclusion,the abrupt shear strength reduction is attributed to the combined effects of suction loss beyond the critical threshold and microscale fabric weakening. These findings enhance the mechanistic understanding of landslide reactivation under hydrological changes.