Abstract: The shear strength of the interface between the fill and the underlying bedrock is an important factor to control the stability of high fill or accumulation slopes. The value of the interface strength parameter is one of the important parameters for the design of high backfills. The effect of the contact surface roughness on the shear mechanical properties of the soil-rock mixture-bedrock contact surface is explored through the systematic large-scale indoor direct shear tests.The test results show that under the action of low normal stress, the shear stress-shear displacement curve shows strain hardening in the early stage and plastic strain in the later stage, and the greater the roughness of the contact surface, the more the contact surface deforms when shear failure occurs. Under the action of high normal stress, the curve shows strain hardening without obvious peaks. Under the same normal stress level, the greater the contact surface roughness, the greater the shear stiffness of the soil-rock mixture-base rock interface. The crushing morphology of the rock at the shear interface includes three types: complete crushing, partial crushing, and surface abrasion. As the contact surface roughness increases, the total number of rock crushing at the shear interface also increases. The shear strength, internal friction angle and apparent cohesion of the contact surface increase with the increase of the roughness of the contact surface. Compared with the internal friction angle, the apparent cohesion of the contact surfaces increases significantly. The roughness of the contact surface has an effect on the width of the shear band, which shows that the larger the roughness of the contact surface is, the wider the shear band is.