Abstract: The mechanical properties of the dam material have an important influence on its stability. In this study, based on the discrete element method(DEM),direct shear tests were conducted on dam materials with different densities and particle grading(fine and coarse-grained materials). From the aspects of material stress-strain curve(macro) and internal stress(micro) information, the mechanical properties of the dam material were revealed. The results show that, from the macroscopic perspective, with the increase of the density of the dam material, the peak strength is enhanced, especially for the coarse-grained material. For different grain gradation materials, the stress-strain curve of the fine-grained material shows obvious strain hardening and shear properties, while coarse-grained materials exhibit strain softening and dilatancy characteristics. From the microscopic perspective, the direction of the internal principal stress of the material gradually shifts as the shear displacement increases, the internal force chains and the internal shear zone gradually concentrateand develops. The particle size distribution of materials determines the distribution of internal contact forces and the mode of transfer. The distribution of stress in coarse-grained materials is mainly distributed along the framework particles, and is evenly distributed in fine-grained materials. The density of the materials affects the contact patterns between the particles inside the material. The greater the degree of compaction, the more conducive to the transmission of occlusal forces between particles, so that the internal stress and the strength of the chain have different degrees of enhancement.