Abstract Coal-fired power plants, cement plants and steel mills are three major CO2 emitters in the coastal areas of China. To examine potential CO2 reduction techniques in coastal areas, we used seawater as a CO2 absorbent with steel slag added to study the CO2 absorption capacity. An online chromatography apparatus was used to determine CO2 solubility in steel-slag-enhanced seawater, and the thermodynamics and kinetics were studied. CO2 was more soluble in seawater with steel slag than that in seawater alone, and as the concentration of steel slag increased, the CO2 solubility increased and mass transfer coefficient increased, favoring CO2 absorption rate by seawater. CO2 solubility increased on average by 115.56% when the steel slag increased by 1.0%. Among steel slag components, MgO had the strongest ability to enhance CO2 capture. CO2 solubility of seawater with 0.4% MgO was 0.0044 at 25 °C and 0.3 MPa. In addition, the combination addition of MgO and CaO had a strong synergistic effect on CO2 capture by seawater. Fe2O3 had a more remarkable capacity to accelerate CO2 capture than other steel slag components. The process was simulated by Aspen Plus to study the optimum operating conditions on the CO2-seawater-steel slag system. The optimum conditions (a liquid-gas mass ratio of 260, absorber temperature of 30 °C, absorber pressure of 1 atm, and steel slag-gas ratio of 0.10) ensured that CO2 absorption capacity was above 90% with low energy consumption and cost. Moreover two-thirds of the CO2 was captured and stored in calcium carbonate form. This process can provide CO2 capture and storage for coastal areas.