<p>The cyclic shear response of the geosynthetic-soil interface under seismic loading is a key factor affecting the seismic performance of reinforced soil structures. To reveal the cyclic and post-cyclic shear characteristics of this interface, cyclic direct shear tests, post-cyclic direct shear tests, and monotonic direct shear tests were conducted using a self-developed large-scale interface shear apparatus. The mechanical behavior and underlying mechanisms of the geogrid-loess interface were investigated. The results show that during cyclic shear tests, the shear stress of the geogrid-loess interface exhibits cyclic hardening under conditions of small shear displacement amplitudes and low compaction degree, while it shows cyclic softening under conditions of high shear displacement amplitudes or high compaction degree. The interface vertical deformation is predominantly contractive, and the cumulative contraction is significantly influenced by the compaction degree. The interface shear stiffness decreases with increasing shear displacement amplitude and water content, but increases with higher compaction degree and normal stress. The damping ratio gradually decreases with increasing number of cycles. The post-cyclic direct shear test results indicate that cyclic shear enhances the confining effect of the geogrid on the soil, leading to an increase in interface cohesion and interface shear strength.</p>