A method and system of fixing carbon dioxide is provided. After metal ion components are extracted from, e.g., natural mineral or steel slag through acid treatment, carbon dioxide is injected to fix carbon dioxide by carbonating the same. Since the procedure of pH adjustment is unnecessary, the reaction is carried out effectively and a continuous process is enabled. Accordingly, the disclosed method of fixing carbon dioxide enables effective removal of carbon dioxide produced from the steelmaking industry, thereby significantly reducing greenhouse gas emission and allowing recycling of the discarded steel slag.