Abstract: In order to investigate the impact of gangue particle size on the mechanical properties and failure characteristics of cemented filling, indoor uniaxial loading tests are conducted on three groups of gangue particle sizes （0-5 mm, 5-10 mm, 0-10 mm） at four different ages. The influence of gangue particle size on compressive strength, stress-strain curves, acoustic emission signals, and failure characteristics of the backfill is systematically analyzed. The results indicate that changes in gangue particle size have minimal effect on the stress-strain curves, with deformation and failure proceeding through compaction, elastic, plastic, and failure stages. An increase in maximum gangue particle size from 5 mm to 10 mm has little impact on early compressive strength but significantly weakens later compressive strength. Moreover, an increase in maximum gangue particle size leads to a significant increase in crack formation within the coal gangue backfill primarily consisting of shear cracks. The gangue fillers with different particle sizes exhibit shear failure characteristics, but as the particle size increases, the integrity of the loaded specimens deteriorates. After the failure of gangue backfills with a particle size range of 5-10 mm, some specimens disintegrate into a large amount of gangue particles. The presence of fine gangue particles can improve the bearing stability of gangue backfills. The research results are helpful in simplifying the gradation of filling gangue in mines and have certain engineering guidance significance for the crushing of gangue in mines.