Abstract: To address the issues of high alkalinity of red mud, complex chemical composition and low resource utilization rate, mine filling materials with red mud in collaboration with fly ash and other multi-solid wastes were prepared, the development law of red mud composites ratio on the compressive strength of backfill was compared. The hydration mechanism of cemented paste backfill was revealed by means of X-ray diffractometer, scanning electron microscope. Through eluviation test and toxic leaching test, the solid alkali mechanism, leaching behavior and toxic ion solidification mechanism of backfill were explored. The results show that the mass ratio of red mud composite material is red mud∶fly ash∶cement=3∶7∶0.4, the compressive strengths of the filling body are 0.76, 1.35 and 1.87 MPa for 3, 7 and 28 d, respectively, and the material cost is 78.08 yuan/t, which satisfy the requirements of mine filling and mining. Under the synergistic interlocking effect of red mud-fly ash-cement, the filling body produces hydration products which are mainly calcium silicate hydrates (C-S-H) gel, ettringite and gmelinite, and the smaller the mass ratio of red mud∶fly ash, the larger the specific surface area of fly ash, the denser the structure of the filling body, and the better the alkali fixation effect. The ettringite and C-S-H gels make the toxic ion leaching concentration of red mud composites meet the requirements of solid waste in through physical sequestration and chemical combination.