Abstract: The mechanical properties of the gravelly sliding zone soils are closely related to the meso-structure of the shear surface. The quantitative characterization of shear surface roughness is of great significance to revealing the meso-mechanism of change of the residual strength in the gravelly sliding zone soils. Based on the reversal direct shear tests and the high-resolution 3D laser scanning technology, the shear surface roughness of the gravelly sliding zone soils is quantitatively characterized, and the relationship between the roughness and the residual strength parameters is analyzed. The results show that there is a positive linear correlation between the friction coefficient and the shear surface roughness. The gravels are the internal factor of the change of the residual strength of sliding zone soils, and vertical stress and water are the external factors. The residual strength of the gravelly sliding zone soils is determined by the contact properties of gravels on the shear surface and the shear surface undulation. The number of occlusive friction contact surfaces depends on the gravel content, and with the increase of gravels content, the occlusive friction interface increases. The maximum particle size of gravel controls the contact area and angle, and the larger the particle size, the greater the occlusive friction. The soil compaction under normal stress and the softening of fine-grained soil due to water are helpful to the transformation of occlusive friction to sliding friction, thus leading to lower residual strength of the sliding zone soils.