As a new type of bulk industrial solid waste, the harmless disposal and large-scale resource utilization of silicon-manganese slag urgently need to be addressed. In this study, silicon-manganese slag was used as precursor and water glass as alkaline activator to prepare alkali-activated silicon-manganese cementitious material. The effects of water glass content, water glass modulus, and slag modification on the macroscopic properties and microscopic properties of alkali-activated silicon-manganese slag cementitious material were investigated. The results show that, within a curing period of 90 d, the alkali activation of silicon-manganese slag cementitious material continues, with the polymerization degree and yield of gel products gradually increasing. The porosity of hardened paste decreases, and the mechanical properties are enhanced. With the devrease of water glass modulus, the early-stage alkali activation reaction is strengthened, the heat release increases, the setting time shortens, and the mechanical performance of hardened paste improves. However, excessively low water glass modulus hinders the later-stage alkali activation. The best mechanical performance of alkali-activated silicon-manganese slag cementitious material is achieved when the water glass modulus is 1.2 and the water glass content is 12% (mass fraction), with strengths of 86.0 and 100.1 MPa at 28 and 90 d, respectively. The incorporation of slag significantly improves the poor early-stage mechanical performance of alkali-activated silicon-manganese slag cementitious material, but excessive addition leads to too rapid alkali activation, causing shrinkage cracks in hardened paste. The optimal slag content is 10%.