Abstract: This study investigates the influence of water content on the shear mechanical properties of gravelly slip zone soil. The soil, collected from an ancient landslide at the junction of the Qinghai-Xizang Plateau and the Sichuan Basin, consists of 16% gravel, 59% sand, and 25% fines, with a plastic limit of 16.1%. Large-scale laminated shear tests were conducted under four water content levels(7.2%, 10.5%, 13.6%, and 16.9%)and four normal stress levels(400, 600, 800, and 1000 kPa). Sieve tests were performed to analyze particle breakage before and after shearing. An exponential strength criterion was adopted to characterize the nonlinear strength behavior. The results showed that: (1)the shear strength of gravelly slip zone soil exhibited nonlinear attenuation with increasing moisture content, with more pronounced attenuation occurring beyond the plastic limit; the strength coefficient displayed a fluctuating "decrease-increase-decrease" trend as water content rose, while the strength index first increased and then decreased, dropping sharply to 0.468 at 16.9% water content; (2)the shear modulus generally decreased with increasing water content, showing the fastest attenuation beyond the plastic limit; inter-ring displacement data revealed that elevated water content shifted the most unfavorable slip surface from a steep inclination to a lower position, with top-ring displacement accounting for less than one-tenth of the total displacement; (3)the gravel particle fragmentation rate was positively correlated with normal stress but was significantly influenced by water content: below the plastic limit, fragmentation resulted from angular interlocking fracture; near the plastic limit, water softening suppressed gravel fracture; above the plastic limit, the fragmentation mechanism shifted to rolling and grinding within the clayey matrix; (4)the correlation between strength/stiffness and particle fragmentation exhibited distinct patterns at the plastic limit: below this threshold, high fragmentation rates corresponded to high strength due to interlocking fracture mechanisms, while above the threshold, high fragmentation rates corresponded to low strength due to grinding failure mechanisms.