Abstract: The interaction characteristics at the interface between reinforcement and soil are crucial factors determining the stability of reinforced soil structures. The particle morphology of the soil significantly influences the interaction at the reinforcement-soil interface. Gravel samples with different particle size ranges (4-6, 8-10, 12-14 mm) and roundness values (0.35, 0.55, 0.75) were prepared using a high-speed centrifugal grinding machine. Monotonic direct shear tests were conducted on the gravel-grid interface using a large-scale direct shear apparatus to investigate the effects of particle morphology on the shear characteristics of the gravel-grid interface. Additionally, a Discrete Element Method (DEM) was employed to establish a direct shear test model of the gravel-grid interface to study the evolution of particle arrangement anisotropy at the reinforcement-soil interface during monotonic shear processes. The results show that during the monotonic direct shear tests, the reinforced soil interfaces exhibited typical shear softening characteristics, with both peak shear stress and residual shear stress decreasing as the roundness of the infill material increased. At the same roundness of the infill, the internal friction angle and apparent cohesion of the interfaces with larger particle sizes were significantly higher than those of the interfaces with smaller particle sizes. Furthermore, for particles with larger sizes and smaller roundness, the principal direction of the normal contact, normal contact force, and tangential contact force exhibited greater deviation angles.