In geosynthetic reinforced soil engineering, the interaction between reinforcement and soil can be effectively enhanced by the use of geosynthetics with additional structures and the setting of coarse-grained soil layers. However, the determination method of the thickness of coarse-grained soil layer is still unclear. In this paper, based on the results of direct shear tests on the high resistance hyperstatic geogrids (HRHG) designed with convex nodes and gravel, a constitutive model of dilatancy at the geogrid-gravel interface under shear hardening condition is established, and the distribution of the additional stress induced by dilatancy in reinforced soil is further investigated. By conducting direct shear tests under different normal pressures (30, 50, 80 kPa). The effects of different coarse-grained soil layer thicknesses (60, 100, 140, 180 mm) on the interaction between reinforcement and soil are evaluated, and the dilatancy distribution at the interface between reinforcement and soil is also compared and analyzed. The results show that the established constitutive model for geogrid-gravel interfacial dilatancy agreed well with direct shear test results, suggesting that it can effectively calculate the shear shrinkage and dilatancy, and the final dilatancy decreases with the increase of normal pressure. The additional stress in coarse-grained soil layer induced by interfacial dilatancy decreases with increasing distance from the geogrid-gravel interfaces, but the scope of dilatancy increases gradually. The increase of the thickness of coarse-grained soil layer can effectively improve the interfacial shear strength, but there exists an optimal layer thickness, which rapidly reduces the increase of interfacial shear strength. The optimal layer thickness decreases with the increment of normal pressare. Through the comparative analysis of the optimal layer thickness and dilatancy ratio, a semi-empirical formula for determining the optimal layer thickness based on the modified dilatancy constitutive model is put forward, which can provide a reference for the design and application of HRHG in practical engineering.