To shorten the setting time and enhance the early compressive strength of magnesium oxysulfate cement (MOSC), the 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 behavior, compressive strength, phase composition, microstructure and pore structure of MOSC prepared using MgO powder with different activities was investigated. The results show that, compared with MgO powder with 65.5% hydration activity, the setting time of MOSC prepared using MgO powder with 75.0% hydration activity is shorter and the early compressive strength is higher. Sucrose is a better choice for MOSC prepared using MgO powder with 75.0% hydration activity. The sucrose with sterically hindered effect improves the fluidity of MOSC slurry, 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 MOSC. In addition, the addition of sucrose is beneficial for the MOSC system to absorb CO2 in the atmosphere to form MgCO3 crystals.