A flow injection on-line analyzer for the determination of free chlorine was designed,and the optimal condition was investigated.Based on the improved N,N-diethyl-1,4-phenylenediamine(DPD) colorimetic principle,the analyzer used flow injection sampling technology,combined with photoelectric conversion,digital signal processing and automatic control technology.It could overcome the defects of DPD spectrophotometry done by manual operation，and perform automated sample analysis.The method was proven to consume less reagent with wide linear range(0.05-6.00 mg/L) and lower detection limit(0.019 mg/L).The recoveries were in the range of 90%-98%.The analyzer was apt to be widely used in monitoring free chlorine in the waterworks and water supply networks with rapid sampling frequency(60 h-1) and higher precision(RSD≤1.2%).