To shorten the setting time and improve the early mechanical property of magnesium oxysulfate cement （MOSC）， MOSC was prepared using magnesia（MgO） powder with different activities， and sucrose was used as a dispersing agent. The effect of sucrose on the setting time， hydration performance， compressive strength， phase composition， microstructure and pore structure of MOSC was investigated. The results show that， compared with samples prepared using MgO powder with 65.5% hydration activity， the setting time of MOSC prepared using MgO powder with 75.0% hydration activity is shorter and its early compressive strength is higher. Sucrose is preferable to MOSC prepared using MgO powder with 75.0% hydration activity. The sterically hindered effect of sucrose improves the fluidity of fresh MOSC paste， prolongs the initial and final setting time， inhibits the formation of Mg（OH）₂， reduces the total porosity and enhances the 28 d compressive strength of hardened MOSC paste. The addition of sucrose is beneficial for MOSC to absorb CO₂ in the atmosphere to form MgCO₃.