The comprehensive utilization of bulk solid waste is an important aspect of Chinas in-depth implementation of sustainable development strategy. Both red mud and coal gangue belong to Chinas bulk industrial solid waste, which have the characteristics of large amount, wide application, outstanding environmental impact and broad utilization prospects. In this paper, a solid waste-based admixture with red mud, coal gangue and lithium slag as raw materials was designed, and the composite cementitious materials were prepared by replacing cement with a solid waste-based admixture. The effects of the mix ratio of solid waste raw materials and the ratio of admixture instead of cement on the mechanical properties, chloride ion corrosion resistance, pore structure and microstructure of composite cementitious materials were investigated. The results show that although the solid waste-based admixture reduces the mechanical properties of the composite cementitious material, it can significantly improve the chloride ion corrosion resistance. The synergistic preparation of red mud, coal gangue and lithium slag optimizes the mineral phase composition of solid waste-based admixtures, so that the admixtures dissolve trace elements and form highly active minerals. In particular, the addition of lithium slag can further enhance the hydration activity of the composite cementitious material, improve its mechanical strength and chloride ion corrosion resistance. This study is expected to provide a theoretical reference for the development and application of solid waste-based admixtures.