In this paper, based on the working principle of the threeelectrode system, a method for measuring the concentration of chloride ion by chronoamperometry is proposed. An online detection system based on threeelectrode system is designed , the quantitative detection of chloride ion concentration in the solution was achieved by collecting the change of current in chronoamperometry, according to the Nernstequation, the temperature compensation formula for measuring the concentration of solution at different temperatures is obtained, and the measured results are calibrated with the standard Sodium Chloride Solution. The experimental results show that the system has the advantages of good stability, high precision and temperature compensation. Compared to the two electrode system, threeelectrode system can accurately measure potential changes of the working electrode, using chronoamperometry can effectively improve the detection sensitivity of the system, so that the detection system has good prospects in the detection of chloride ions in industrial water.