Effects of n(MgO)/n(MgSO4), n(H2O)/n(MgSO4) on the properties and micro structural mechanism of magnesium oxysulfate(MOS) cemented materials were investigated. The results show that the most optimal mechanical properties are obtained when n(H2O)/n(MgSO4) is fixed to be 19∶1 and n(MgO)/n(MgSO4) equals to 14∶1. Micro structural analysis shows that once n(MgO)/n(MgSO4) is over 14∶1, the content of residual MgO and Mg(OH)2 in hardened paste of MOS will increase, which will loosen the structure of the hardened MOS and reduce its mechanical properties; With the increase of n(H2O)/n(MgSO4) the formation of basic magnesium sulfate phase in hardened MOS will be retarded, resulting in a large amount of pores in microstructure and decrease of mechanical properties, when n(MgO)/n(MgSO4) is 14∶1.