The properties of magnesium oxysulfate (MOS) cement with different granite powder (GP) contents with CO₂ curing were studied. The hydration products and microstructure of MOS cement were analyzed using X-ray diffraction (XRD), simultaneous comprehensive thermal analyzer (TG-DSC), scanning electron microscope (SEM) and mercury porosimeter (MIP). The results shows that the strength retention coefficient of MOS cement containing 40% GP reached 1.12 with standard curing for 28 days followed by 28 days of CO₂ curing; when MOS cement was CO₂-cured for 28 days and then immersed in water for 120 days, the strength retention coefficient reached 0.94, which was 123.8% higher than that of blank samples. MgCO₃·zH₂O crystals were generated after MOS cement underwent CO₂ curing, which had a positive effect on the mechanical properties and water resistance by reducing matrix expansion caused by MgO hydrating to Mg(OH)₂. GP increased the matrix density, reduced the CO₂ capture rate, and decreased CO₂ erosion of MOS cement by optimizing the pore structure. The presence of GP and magnesium carbonate phases improved the compressive strength of MOS cement after carbonation and immersion.