Abstract: The sand-geomembrane interface is a potential weak interface in landfill liner systems, and its shear behavior plays an important role in landfill stability. To reveal the micromechanical mechanisms governing the shear behavior of the sand-geomembrane interface, a discrete element model of the sand-geomembrane interface direct shear test was established in this study. The model captured the post-peak softening behavior of the interface, and micromechanical analyses were conducted in terms of particle contact types, contact force distributions, and the evolution of sliding contacts based on the calibrated model. The results show that the macroscopic shear resistance of the interface is mainly carried by the cross-interface contact network between sand particles and geomembrane particles, and the soil-geomembrane contact force shows good consistency with the macroscopic shear force in terms of its evolution trend. The soil-geomembrane contact force exhibits a markedly non-uniform distribution, with a small number of strong contacts carrying most of the interface shear resistance, indicating that the formation of interface strength is characterized by localized force transmission. Further analysis shows that both the sliding contact ratio and the proportion of strong sliding contacts decrease during the post-peak stage, while the remaining strong sliding contacts still carry most of the sliding contact force. This suggests that interface softening is closely related to the shrinkage of the sliding contact network and localized load-bearing behavior. As the surface roughness of the geomembrane increases, the residual sliding contact ratio gradually decreases and follows a power-law relationship with surface roughness, indicating that the interface shear mechanism gradually transforms from geomembrane-surface sliding dominated behavior to particle interlocking and internal shear within the sand.