Abstract: The use of steel slag to capture and sequester CO₂ is an effective method for coupling solid waste resource utilization with carbon emission reduction. This process can also neutralize f-CaO in the slag, thereby improving its volumetric stability. However, owing to the dense structure of steel slag and the inert forms of CaO as silicates, its direct carbonation performance is limited. Therefore, ball milling modification with potassium chloride (KCl) addition was applied to the basic oxygen furnace (BOF) slag to enhance its surface chemical reactivity and, in turn, improve its carbonation performance. This study presents a systematic analysis of the effect of ball milling modification with KCl addition on the carbonation performance of BOF slag, combining experimental analysis with theoretical calculations. In the experiments, the parameters of CO₂ uptake and carbonation conversion were used to evaluate the influence of ball milling with KCl using a fixed-bed reactor, X-ray diffraction, scanning electron microscopy, and thermogravimetric analysis. In the theoretical calculations, a first-principle computational approach based on density functional theory was used to gain deeper insights into the effect of K on CO₂ adsorption at the microstructural level. The experimental results indicated that ball milling with KCl led to Ca-enrichment on the surface of the BOF slag particles, reducing the diffusion resistance of CO₂. Furthermore, an optimal amount of KCl facilitated the dispersion of BOF slag particles during ball milling, resulting in more micropores and mesopores that promoted CO₂ diffusion. From an electronic structure perspective, the adsorption of K possibly altered the charge distribution on the surface of BOF slag particles and created electronic structural defects, thereby providing active sites to facilitate the reaction with CO₂. Consequently, an appropriate amount of KCl enhanced the CO₂ uptake and carbonation conversion of BOF slag, reaching a maximum of 46.3 g·kg^–1 and 12.5% under the condition that the mass fraction of KCl is 3%. However, excessive KCl might lead to 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 resulted in the substitution of K ions for Ca ions in the CaCO₃ lattice during the carbonation process, leading to the localized formation of K₂CO₃. This increased the instability of the CaCO₃ lattice structure, thus promoting the thermal decomposition of CaCO₃. Theoretical calculations showed that the adsorbed K on the C₂S (010) surface enhance the stability of CO₂ adsorption, with a relatively low adsorption energy of −0.795 eV, indicating that the presence of K strengthened the CO₂ capture capacity of C₂S, thereby improving the carbonation performance of BOF slag. Comprehensive experimental and theoretical calculations showed that ball milling modification with KCl not only improved the carbon sequestration performance of BOF slag but also eliminated the presence of f-CaO in the slag, offering new insights into the resource utilization of BOF slag with alkali metal waste.