Abstract: Red mud is a solid waste produced in the process of bauxite refining alumina, with high alkali content, and its treatment methods are mainly stacking and ocean dumping, which not only occupy a large amount of cultivated land and pollute land and water sources, but also have high safety risk. The preparation of red mud-based filling materials to fill the underground goaf can improve the utilization rate of mineral resources and reduce the harm of red mud to the environment, which has the effect of killing two birds with one stone. In view of the problems of low utilization rate of bayer red mud in mine filling system, low strength, bleeding and shrinkage in filling materials slurry with low concentration, the effects of the addition ratio of fly ash, desulfurization gypsum, lime and initiator on the early strength and volume stability were studied in this paper. Scanning electron microscope- energy dispersive spectroscope (SEM-EDS) and X-ray diffraction (XRD) were used to analyze the hydration mechanism of the filling materials. The results show that when the ratio of red mud to fly ash is 4∶6, the mechanical properties of the filling material are the best. Desulfurized gypsum promotes the formation of ettringite. Lime promotes the pozzolanic effect of fly ash. The composite activator can accelerate the hydration process of red mud and fly ash. All of this enhance the red mud backfill strength. The filling materials 28 d compressive strength is 3.35 MPa, and the initial and 60 min fluidity are above 200 mm. Microscopic test results show that the hydration products of hardened paste are ettringite, lawsonite, silica aluminate gel, which fill the pores and improve the strength of slurry. Through adding activator, activating red mud activity and designing low concentration filling material, it is the direction of mass and green utilization of red mud, desulfurization gypsum and other solid wastes. The utilization ratio of solid waste of red mud filling materials reaches 92%, no bleeding, no shrinkage, and has high economic value and environmental value.