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Monitoring the service condition of concrete structures requires the quantitative assessment of properties and corrosion rate of structural steels surrounded by concrete.A multi-cell sensor system that included a reference electrode, a chloride content sensor, a macrocell current unit and an electrical resistance measurement unit was developed.This system provided the following important electrochemical data in the cover-zone concrete on site: open circuit potential, macrocell current from anodes to cathode,chloride profile, concrete resistance and corrosion rate of built-in anodes.The experimental results show that the macrocell current increases when the chloride content in concrete is higher.Thus, monitoring the chloride content is a good method for monitoring the corrosion state.The chloride ion content and cover depth are the key factors that affect the electrical resistance of concrete.Without considering the temperature and time, a simplified model of the instantaneous corrosion rate of steel rebar in a concrete structure based on the measured chloride contents and concrete resistance was proposed.The test results further prove the reliability of this simplified predicting model.