Steel slag was used as a low-cost feedstock to prepare CaO-based sorbents for CO2 capture by acidification treatment, and the acidification process was optimized. Four main acidification parameters (i.e., extraction time, extraction temperature, acetic acid concentration, and solid/liquid ratio) were investigated. The solid/liquid ratio and extraction time are the most important factors that affect the CO2 capture capacity and stability of the sorbents. The CO2 sorption performance of optimal steel-slag-derived sorbent is more stable than that of naturally occurring limestone, due to the low Si/Ca ratio and the presence of MgO with high antisintering ability. CaO-based pellets with high resistance to attrition and compression were produced by extrusion of the steel-slag-derived sorbent powders.