To study the mechanical behavior of gangue filling materials in depth, through the physical compression testing of red-sandstone gangue of different sizes and numerical simulation of grain flow, the bearing characteristics and deformation and failure rules of gangue compacting were comprehensively compared and analyzed from macro-scopic and microscopic perspectives. The results show that under the same vertical load, the settlement decreases monotonously with the decrease of grain size, and the filling effect is significantly enhanced. Structure of force chain between particles evolves from the & ldquo;polygon & rdquo; structure to the & ldquo;triangle & rdquo; structure, and small particle forms a stable structure earlier than large particle. With the gradual increase in axial strain, the main propagation direction of the force chain is the vertical direction. The distribution law of the tangential contact force of large gangue in the later stage of loading is basically the same as that of small gangue in the early stage (c) 2021 Elsevier B.V. All rights reserved.