Broken gangue from the collapsing roof of a longwall coal panel can be used as the backfilling material for goaf backfilling. The particle size gradation has important influence on the compressive deformation of the gangue backfilling material and particle breakage. The compressive deformation of the gangue backfilling materials at 4 different particle size grades is simulated by PFC3D in this paper. The law of the compressive deformation of the gangue backfilling material, the particle cluster distribution and the variation of the gangue block shape are analyzed. The microscopic mechanism of the compressive deformation of the gangue backfilling material is investigated based on the force-chain distribution. The results indicate that when the gangue backfilling material has a small particle size, the specimen has low porosity and a few amounts of the broken particles can fill the pores of the material. When the particles have relatively large sizes, the specimen has relatively high porosity and the broken particles are unable to completely fill the pores. The loading conditions of the particles can only be changed by the frame structure formed by the large particles. If the proportion of the particle size grade is reasonable, a frame structure forms due to the large particles and the small particles with various particle sizes can fill the pores of the backfilling material. This enhances the deformability of the gangue and decreases the compressive deformation of the backfilling material. After backfilled into the goaf, the backfilling material bears the overburden pressure, which means the process of roof convergence is the process of compressive deformation of the backfill body. That is to say, the stiffer the backfilling material, the better the control effect. The importance of the reasonable proportion of the particle size grade is explained from the aspect of microscopic breakage of the particles, which provides a scientific basis for backfill mining. (C) 2019 Elsevier B.V. All rights reserved.