Abstract: Utilizing steel slag to capture and sequester CO2 is one of the effective methods to realize the coupling of solid waste resource utilization and carbon emission reduction. It can also neutralize f-CaO in the slag, enhancing the volumetric stability of the steel slag. However, due to the dense structure of the steel slag and the inert forms of CaO as silicates, its direct carbonation performance is negligible. Therefore, ball milling modification with potassium chloride (KCl) addition was employed on basic oxygen furnace (BOF) slag to enhance its surface chemical reactivity, and further improving its carbonation performance. In this paper, a systematic analysis of the effect of ball milling modification with KCl addition on the carbonation performance of BOF slag was conducted, combining experimental analysis with theoretical calculations. In experiments, the parameters of CO2 uptake and carbonation conversion were adopted to give a judge on the influence of ball milling with KCl through a fixed-bed reactor, XRD, SEM and TGA. In theoretical calculations, the first-principles computational approach based on the density functional theory (DFT) was applied to give a deep insight into the effect of K on CO2 adsorption from the microelectronic structure. Experimental results indicate that ball milling with KCl leads to Ca-enrichment on the surface of BOF slag particles, reducing the diffusion resistance of CO2. Furthermore, an appropriate amount of KCl may help to disperse BOF slag particles during the ball milling process, and form more micropores and mesopores, which are beneficial for the diffusion of CO2. From an electronic structure perspective, the adsorption of K may alter the charge distribution on the surface of BOF slag particles and create electronic structural defects, thereby providing more active sites to facilitate the reaction with CO2. Consequently, an appropriate amount of KCl can enhance the CO2 uptake and carbonation conversion of BOF slag, reaching a maximum of 46.3 g·kg-1 and 12.5% under 3% KCl condition. However, excessive KCl may lead to the collapse or blockage of the pore structure and cover the active sites on the surface, thereby reducing the carbonation performance of BOF slag. Additionally, the attachment of K ions to the surface of BOF slag particles results in the substitution doping of K ions for Ca ions in the CaCO3 lattice during the carbonation process, resulting in the localized formation of K2CO3. This can increase the instability of the CaCO3 lattice structure and promoting the thermal decomposition of CaCO3. Theoretical calculations found that the adsorbed K on the C2S (010) surface can enhance the stability of CO2 adsorption, accompanied by a relatively lower adsorption energy of -0.795 eV, indicating that the presence of K strengthens the CO2 capture capacity of C2S, thereby enhancing the carbonation performance of the BOF slag. Comprehensive experimental and theoretical calculations have shown that the ball milling modification with KCl can not only improves the carbon sequestration performance of BOF slag but also eliminates the presence of f-CaO in the slag, providing new insights for the resource utilization of BOF slag with alkali metal waste.