<p>The stability of reservoir-bank deposit landslides developed in steep mountainous gorges is a key concern for the operation of hydropower infrastructure. A major difficulty in their stability evaluation lies in reliably defining the shear strength of potential sliding surfaces. Taking the Huangcaoba No. 2 landslide on the Jinsha River in Southwest China as a case study, large-diameter ring-shear tests were conducted with a newly developed apparatus under varying normal stress, water content, shear rate, and medium-gravel (10–20&#xa0;mm) content. A Weibull-based statistical damage model was then employed to investigate the damage-evolution mechanism. The results indicate that water content is the dominant factor controlling both the peak and residual shear strength along the potential sliding surface. Within the tested water-content range and under consolidated drained ring-shear conditions, increasing water content mainly reduced shear resistance by weakening suction-related bonding and enhancing lubrication at particle contacts. The shear rate primarily governs the progression of damage evolution, with higher shear rates substantially shortening the displacement required to reach the residual state. Within the tested range of 5–13%, the influence of medium-gravel content on the overall mechanical response is relatively limited. In the proposed damage-evolution equation, the model parameters respectively quantify material heterogeneity and the degree of strain softening, thereby establishing a quantitative link between macroscopic strength degradation and microscopic damage evolution. These findings provide reliable experimental results and a theoretical basis for determining the shear strength of potential sliding surfaces in reservoir-bank deposit landslides and for stability evaluation.</p>