The effect of the molar ratio（n（MgO）∶n（MgSO₄）∶n（H₂O）） in the raw materials on the mechanical properties and deformation characteristics of modified magnesium oxysulfide（MMOS） cement was studied， and the mechanism was analyzed by testing techniques， such as X-ray diffraction（XRD）， scanning electron microscope（SEM）， Fourier transform infrared spectroscopy（FTIR） and thermogravimetric analysis（TG）. The results show that the compressive strength and flexural strength of MMOS cement matrix shows an increasing trend with the increase of water-sulfur ratio and oxygen-sulfur ratio. The specimen with a molar ratio of 10∶1∶12 has the highest compressive and flexural strength. The deformation of MMOS cement specimens with different molar ratios during the period from completion of pouring to 56 days of curing is mainly expansion deformation. The total deformation of MMOS cement specimens shows a decreasing trend with the increase of water-sulfur ratio and oxygen-sulfur ratio， while the autogenous deformation shows a decreasing trend with the increase of water-sulfur ratio and a first increasing and then decreasing trend with the increase of oxygen-sulfur ratio. The MMOS cement specimens with different molar ratios exhibit differences in expansion deformation， mainly due to the different contents of hydration products Mg（OH）₂， 5·1·7 phase （5Mg（OH）₂·MgSO₄·7H₂O） and unreacted MgO phase in the hardened matrix. When the content of Mg（OH）₂ decreases and the content of 5·1·7 phase increases， the expansion deformation phenomenon of MMOS cement specimens weakens， while their flexural strength and compressive strength are improved.