Magnesium oxychloride cement (MOC) has some advantages like light weight, early strength, low thermal conductivity, and fire resistance, but poor water resistance restricts the application of MOC in civil, construction and other fields. This paper was to improve the water resistance of MOC by using soluble sulfates containing different metal cations as additives. The effect of soluble metal cations on the setting time and compressive strength of MOC was investigated. The phase composition, microstructure and pore structure of modified MOC were characterized by X-ray diffraction, scanning electron microscopy, and mercury intrusion porosimetry. The results show that Al3+, Fe2+, Cu2+ ions and free OH- ions in the MOC slurry can preferentially form a precipitate, inhibiting the formation of Mg(OH)2, delaying the hydration of MgO, prolonging the setting time, reducing the total porosity of the MOC matrix, and improving the water resistance of MOC. In addition, SO42- ions can also improve the stability of 5Mg(OH)2·MgCl2·8H2O after soaking in water through the adsorption and coordination with Mg2+ ions in 5Mg(OH)2·MgCl2·8H2O.