Aiming at the common detection problems of residual chlorine in water, the working principles and manufacturing method of residual chlorine sensor based on threeelectrode system have been introduced and studied, and this paper also proposed the design of online measurement instrument for residual chlorine. Based on Reference Electrode which has the ability of feedback control to make Working Electrode potential be stable and Counter Electrode made of spiralshaped silver wire, the newstyle trielectrode electrochemical sensor has better performance in wide measuring range and chemical reaction than bielectrode sensor. Experimental test result indicates that the online measurement system using residual chlorine sensor with three electrodes has stable performance in the 0~10 mg/L measurement and its linearity error is ±20.77 μg/L, which can be used for the online detection of free chlorine in drinking water，boiler feed water and sewage treatment.