Sodium dihydrogen phosphate (NaH2PO4) and potassium dihydrogen phosphate (KH2PO4) were selected as additives for magnesium oxysulfate (MOS) cement. The phase composition and the microstructure of MOS cement were characterized using X-ray diffraction (XRD), thermogravimetric analysis (TG-DSC), Flourier transform infrared spectroscopy (FT-IR), mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). It is found that both NaH2PO4 and KH2PO4 lead to an increase in the compressive strength and an improvement in the volume stability of MOS cement. The XRD, MIP and SEM results show that the addition of NaH2PO4 or KH2PO4 does not change the phase composition of MOS cement but promotes the formation of strength phase of 5Mg(OH)2·MgSO4·7H2O (5·1·7 phase). This phase brings a considerable improvement in the microstructure of MOS cement, which has a positive effect on the properties of MOS cement.