Abstract: Carbonaceous slate, widely distributed in colluvial landslides across the Bailong River Basin, undergoes significant hydro-mechanical degradation when weathered and saturated, transforming into weak slip zone soils. However, the mechanisms linking water-rock interactions to strength deterioration and landslide reactivation remain unclear. This study investigates the Jiangdingya landslide through disintegration tests, rainfall-simulated wet-dry cycles, and in-situ shear tests under varying normal stresses (100~600 kPa). The results reveal that: (1) Hydrophilic clay mineral expansion induces complete disintegration within 332 minutes, while 20 wet-dry cycles reduce uniaxial compressive strength by 12.6% due to structural fatigue from cyclic swelling and shrinking. (2) Shallow slip zones (<37.5 m depth) exhibit strain softening under low normal stress (<300 kPa), contrasting with strain hardening in deep zones under high stress (>300 kPa). (3) Rainfall modeling demonstrates accelerated crack propagation and 14.58 mm of frontal displacement within 48 hours, driven by a self-reinforcing "deterioration-infiltration" feedback loop. (4) Landslide reactivation is governed by coupled shallow creep unloading and deep stress redistribution, forming a cyclic mechanism of strain softening and re-softening. These findings provide critical insights for landslide mitigation and engineering practices in colluvial slopes.