In order to improve the water resistance of magnesium oxysulfate （MOS） cement， hydromagnesite or magnesite was added into MOS cement. The effects of hydromagnesite and magnesite on the compressive strength， phase composition and content of hydration product， and microstructure of MOS cement were analyzed. The results show that the addition of hydromagnesite and magensite is conducive to the water resistance of MOS cement. When the dosage of hydromagnesite or magensite is 10 %， the compressive strength of MOS cement increase to 77.9 MPa and 76.7 MPa， and the softening coefficient increases by 26.3% and 15.0%， respectively. With the increase of the dosage of hydromagnesite or magnesite and the extension of curing age， the content of MgO decreases， and the soundness problem of MOS cement is effectively alleviated. Hydromagnesite and magnesite can react with the residual MgO in MOS cement to form the MgO-CO₂-H₂O system， which is stable in water and compacts the matrix， thus improving the compressive strength and water resistance of MOS cement.