Abstract: Based on the background of large amount of solid waste alkali slag, high treatment cost and environmental pollution, the feasibility of using alkali slag to improve mechanical properties of gangue cemented backfill material was discussed. In this paper, the basic physical and mechanical properties of alkali slag are tested, and the influence law and strength mechanism of alkali slag content and curing age on mechanical properties of cemented filling materials are studied. The results show that: with the increase of alkali slag content in the range of 0%-12%, the strength of the backfill in the early stage, the middle stage and the later stage show the law of first strengthening and then decreasing; the optimum mix ratio of alkali slag cemented backfill material is alkali slag∶ fly ash∶ lime∶ cement∶ gangue∶ used aggregate=6%∶34%∶10%∶2.5%∶24%∶23.5%, and the early strength and late strength of the material increase up to 449% and 187%; C-S-H cementation coexists with N-A-S-H cementation in the slurry system of alkali slag cemented backfill material, and N-A-S-H cementation has stronger cementation performance, which reduces the connectivity of pores. An appropriate amount of alkali slag can effectively improve the strength of the material. The cemented filling material prepared by alkali slag can realize energy saving and emission reduction, and its technical, economic and social benefits are remarkable, and it has good engineering application value.