To reveal the difference of corrosion mechanism of steel bars in concrete under Cl?傆b and SO42?傆b action, and provide the theoretical basis for the control of concrete corrosion resistance under chloride and sulphate attack, the electrochemical behavior of reinforcement subjected to Cl?傆b and SO42?傆b in concrete pore solutions was investigated, and the competitive adsorptions of corrosive ions (Cl?傆b, SO42?傆b) and passive ions (CaOH+, OH?傆b) on the Fe (100) surface were analyzed by the density functional theory based on the first-principles. The results show that the corrosion potential and polarization resistance of steel reinforcement in concrete pore solution with SO42?傆b both are lower than those in concrete pore solution with Cl?傆b, indicating that the steel reinforcement corrosion occurs more easily in concrete pore solution with SO42?傆b. The inhibition effect of SO42?傆b on the reaction of OH?傆b and iron surface is slightly stronger than that of Cl?傆b. Moreover, SO42?傆b can promote the elongation of Ca-OH bond, and block the interaction of the OH group in CaOH+ and surface Fe atom, showing that the inhibition effect of SO42?傆b on the reaction of CaOH+ and iron is stronger than that of Cl?傆b. The results reveal the corrosion mechanism of steel reinforcement under the attack of Cl?傆b and SO42?傆b in concrete.