Silicic acid was used as a modifier to magnesium oxysulfate (MOS) cement to improve the water resistance of MOS cement. The effect of silicic acid on the mechanical properties, volume stability, and water resistance was investigated. The phase composition, microstructure, and ion leaching concentration were investigated. The co-existence of magnesium silicate hydrate (M-S-H) gel with MOS system was also analyzed. The results show that the addition of silicic acid enhances the mechanical properties and improves the volume stability of MOS cement through promoting the formation of 5Mg(OH)2·MgSO4·7H2O crystals and M-S-H gel. When MOS cement containing silicic acid immerses in water, the residual silicic acid continues to react with free Mg2+ and OH- to form M-S-H gel. Meanwhile, 5Mg(OH)2·MgSO4·7H2O crystals and M-S-H gel coexist in MOS cement, resulting in an improvement in the water resistance of MOS cement.