In order to improve the strength stability and water resistance of magnesium oxy-sulfate (MOS) cemented materials, the influence of sodium silicate with different content on performance and mechanism of MOS cemented materials was investigated and also TG, XRD and SEM techniques were adopted to analyze the corresponding influencing mechanism. The results show that sodium silicate can effectively reduce the dispersion of MOS cemented materials in water, and obviously improve the long-term strength stability. When the dosage of sodium silicate is 0.5%～1% of light-burnt MgO mass, and strength loss rate of hardened MOS cement paste in the water is the lowest, compressive and flexural strength reaches the highest. The micro-meso structural analysis results show that in the hardened system of MOS cemented materials with 0.5% sodium silicate, the content of fibrous basic magnesium sulfate phase increases, that of flake Mg(OH)2 phase decreases obviously, and the corresponding hardened structure tended to be more compact. The analysis of the hydration reaction of MOS cemented materials shows that sodium silicate can accelerate the endothermic reaction in the system of MOS cemented materials, increase the concentration of H+ in the reaction system and eventually produce more stable alkaline magnesium sulfate with better gelatinization.