In order to reasonably evaluate the contents of chloride ion for reinforced concrete corrosion, effects of carbonation on chloride ion concentration distribution inside hardened cement paste were investigated and the corrosion characteristics of reinforced concrete under the combined action of carbonation and chloride were clarified. The results show that under carbonation Friedel salt diffraction peaks abound in non carbonation zone of hardened cement paste containing chloride while this kind of diffraction peaks could not be found in carbonated zone. During carbonation process the chloride ion from decomposition of Friedel salt migrates to the non carbonation area and accumulated, chloride ion concentration deceases in carbonation zone and significantly increases in carbonation interface; chloride ion distribution in hardened cement paste containing 10% chloride ion is relatively uniform before carbonation and its relative concentration peak value is 68. After two weeks carbonation, the maximum peak value becomes 151 and four weeks carbonation later this value reaches 298. Thus the combined action of carbonation and chloride salt increasingly reduces the initial critical chloride ion concentration for reinforced concrete corrosion and increases the chance of steel corrosion in carbonation frontier, thus steel corrosion rate is accelerated.