Abstract: Aiming at studying the development of the microstructure in the interface and evaluating the reinforcement effect of the particle size, elliptical group particles are generated with the assistance of “clump ”method to factually simulate the shape properties of sand particles. A three-dimensional DEM model is established, and its validity is verified by comparing the laboratory and numerical results. The mechanical responses such as pull-out resistance, strain of geogrid as well as local void ratio are systematically analyzed, and the laws of development are revealed. The results from the pull-out tests show that the apparent cohesion greatly increases, while the friction angle varies little after the original filler is partly replaced by coarse aggregate. The macro and micro analyses reveal that to a greater extent the positions of particles in the replacement systems chang and that the increment of total pull-out resistance is mainly attributed to the increase of friction resistance. The research results may provide a new understanding for the mechanism study of the reinforcement interface from a mesoscopic view.