The basic oxygen furnace steelmaking slag (SL) is employed for CO2 mineralization. Response surface methodology and the central composite design were employed in determining the optimal condition. The optimization goal in this paper has been set for maximum CO2 capturing. It was found that the reaction temperature and CO2 pressure and their combination were significant. A quadratic model was developed for process optimization and statistical experimental designs. The CO2 capture capacity could reach 126 g CO2/kg SL at optimal condition. The increased reaction temperature will lead to an obvious decrease of CaCO3 and increase of MgCO3. If deployed, this optimized indirect CO2 mineral sequestration process could permanently capture 252,000 tons of CO2 per annum based upon current 2 million tons of SL productivity per annum.