The effect of the carbonation on the properties of the steelmaking slag-slaked lime (SmS-SL) mortar and its mechanism were mainly investigated. Furthermore, the environmental benefits of the SmS-SL mortar were discussed as well. Results indicated that carbonation could strengthen the SmS-SL mortar, reduce its drying shrinkage and improve its soundness. With the increase in the steel slag content, the strengthening effect of carbonation on the SmS-SL mortar is enhanced, but its dry shrinkage reduction effect is weakened. The SmS-SL mortar using steel slag exhibits significant environmental benefits. Its CO2 sequestration-efficiency and steel slag-recycle efficiency respectively reaches 6.4% and 27.8%.