Abstract: Cemented backfill mining using coal gangue is an important technical approach for green coal mining. The particle size of coal gangue significantly influences the flow behavior of backfill slurry and the structural stability of the consolidated backfill. In this study, slump tests on backfill slurry and uniaxial compression tests on backfill specimens were conducted to systematically investigate the effects of coal gangue particle size on slurry flowability and the bearing capacity of the backfill mass. The results show that the average particle size of coal gangue exhibits opposite trends with respect to the slump percentage of the backfill slurry and the uniaxial compressive strength of the backfill. Under constant gangue-cement ratio and mass concentration, a well-graded particle size can substantially improve both the slurry flowability and the compressive strength of the backfill. When the gangue-cement ratio is 4∶1 or 7∶1, the optimal average particle size is 1.92 mm; when the ratio is 10∶1, the optimal average particle size ranges from 3.99 to 4.74 mm. As the coal gangue particle size decreases, the internal structural integrity of the backfill is enhanced, and under the same deformation condition, the failure mode gradually transitions from tensile fracture to a combined tensile–shear fracture.