In order to study the corrosion resistance of magnesium oxy-sulfate （MOS） cementitious materials， the electrochemical impedance， polarization curve and corrosion area ratio of passive and de-passive steel bars were investigated under natural curing， carbonation， chloride salt and combined action of carbonation and chloride salt. The results show that the impedance of steel bars under single carbonation and chloride salt condition is lower than that in natural curing. With the increased age， the impedance of passive steel bars decreases first and then increases， while that of de-passive steel bars decreases continuously. Under the combined action of carbonation and chloride salt， the electrochemical impedance of steel bars in MOS cementitious materials is further reduced， which reflects the superposition effect of combined action of carbonization and chloride on the corrosion degree of steel bars in MOS cementitious materials. The corrosion degree of de-passive steel bars is higher than that of passive steel bars under all curing conditions.