Abstract: Gangue particle size and mass concentration have an important influence on the performance of backfilling materials. The size grading of gangue was characterized by the fine gangue content (the percentage of 0−5 mm gangue in the total gangue), and the influence of the fine gangue content, concentration and their interaction on the flow performance, rheological properties, mechanical properties, ultrasonic wave velocity and microstructure of backfill was analyzed. The results show that with the increase of fine gangue content and concentration, the slurry flow, bleeding rate and the total porosity decreased, while the rheological parameters, uniaxial compressive strength, modulus of elasticity, splitting tensile strength and ultrasonic wave velocity increased, making the backfill more compact. The slurry belonged to Bingham fluid, and the fluctuation of slurry shear stress curve decreased and the homogeneity became better with the increase of fine gangue content. In the interaction of two factors on the rheological parameters and mechanical properties, the increase of one factor promoted the enhancement of the other factor, but the influence of concentration was greater than that of fine gangue content. The response surface of each property to the interaction term could be fitted as parabola function surface. The splitting tensile strength was mainly 0.11-0.12 times of uniaxial compressive strength, and a significant linear positive correlation lied between them. The uniaxial compressive strength was positively correlated with ultrasonic wave velocity and the content of micropores and capillary pores, but negatively correlated with total porosity and the content of large pores and multi-damaged pores, with the fitting degree greater than 0.9. The findings demonstrate that the performance of acoustic wave velocity and microstructure in describing the influencial mechanism of these two factors on backfill strength.