The corrosion behaviour of steel in concrete was studied under the interaction of carbonation and chloride ion. The electrochemical parameters of corrosion potential（Ecorr） and corrosion current density （Icorr） were tested in the corrosion process of steel by electrochemical instrument. The effects of carbonation and chloride ion on the steel corrosion behaviour were analyzed combined with the variation of chloride ion concentration and pH value in mortar around the steel. The results show that the aggressive chloride ion can accelerate the steel corrosion process; while the carbonation can retard the steel corrosion process in the interaction of carbonation and chloride ion.