Abstract: The critical chloride ion concentrations of corrosion of steel bar in simulated concrete pore solution (saturated calcium hydroxide solution) under constant potential and constant current cathodic protection were studied by means of potential and current change and surface observation, combined with electrochemical impedance spectroscopy. The results show that under the cathodic protection of 10 mA/m² current density, the critical chloride ion concentration leading to corrosion of steel bar was 0.03 mol/L, which was the same as that without cathodic protection. When the cathodic protection current was large enough (greater than 10 mA/m²), the steel bar was not corroded in the simulated pore solution, and there was no critical chloride ion concentration. In the under-protective state of constant potential cathode protection, with the positive shift of protection potential, the critical chloride ion concentration for corrosion of steel bars increased gradually. When the chloride ion concentration exceeded the critical chloride ion concentration, the steel bar would dissolve, because the potential was negative to the protection potential due to the localized corrosion of the steel bar; based on the safety of the structure, the cathode protection of the steel bar in the simulated pore solution should be controlled by constant current.