Abstract: In order to study the effects of fly ash particle size on the properties of high concentration cemented filling materials, the undisturbed fly ash was ground and analyzed, and the fly ash particles with different particle sizes were obtained. Through the test of void fraction of fly ash - cement - coal gangue mixed material, the effect of fly ash particle size characteristics on the void fraction of mixed material porosity and the relationship between the void fraction and properties of slurry including the rheological characteristics, the slump and the uniaxial compressive strength were studied. The results show that there is a good linear relationship between the slump of mixed material and the void fraction; the yield stress and plastic viscosity of the slurry decreased with the decrease of the particle size of the fly ash; the uniaxial compressive strength increased with the decrease of the particle size.