Abstract: To investigate the effect of water content on the shear mechanical properties of the soil-rock mixture interface, large-scale direct shear tests were conducted on the soil-rock mixture interface under varying conditions of water content and roughness. The test results reveal that the specimen exhibits plastic strain failure at medium and low water contents. At high moisture contents, strain hardening failure occurs. Roughness primarily influences the displacement required for shear stress-displacement curves to reach plastic strain. The shear strength of the contact surface is affected by water content, roughness, and normal stress. Shear strength increases with higher normal stress and roughness, but decreases with increased water content. The order of significance is normal stress>roughness>water content. Cohesion increases and decreases with the increase of roughness and water content. The internal friction angle experiences a slight increase and decrease with the rise of roughness and water content.