In order to investigate the effect of mineral admixtures on modified magnesium oxysulfate cement and its mechanism, different amounts of fly ash and mineral powder were blended into the modified magnesium oxysulfate cement, and the mechanical properties, water resistance and acid resistance were studied. The composition and microstructure of the material phase were characterized and analyzed by X-ray diffraction and scanning electron microscopy, respectively. The results show that the incorporation of fly ash will increase the 3 d strength of modified magnesium oxysulfate cement, and the later strength decreases, its 28 d compressive strength loses 14.7% compared with the reference group when the fly ash dosage is more than 20% (mass fraction). The incorporation of mineral powder has little effect on the preliminary strength of modified magnesium oxysulfate cement, and makes the later strength decrease, and the 28 d strength loss rate of the cement is as high as 17.3% when the mineral powder dosage is 30% to 40% (mass fraction). Both fly ash and mineral powder are able to enhance the water resistance and sulfuric acid corrosion resistance of modified magnesium oxysulfate cement, where the corrosion resistance of cement is enhanced with the increase of fly ash admixture, and the best dosage of mineral powder with the best sulfuric acid corrosion resistance is 20%.