In our previous study, a low cost detector with a photodiode for chemical oxygen demand(COD) analysis using luminol-H2O2-Cr3+ chemiluminescence reaction andmanual syringe injectionmode had been proved. Herein, the previously method was improved applying a flow injection system. The effects caused by Br- andAg+ ions were investigated in detail. Moreover, the conditions for COD determining such as pH, concentrations, flow rate, interference and sample proceeded procedures were also optimized. With the developed instrument, the practical detection limit of 2.1mg/ L COD with the linear range of 2.1-600mg/L was achieved. The data for environmental monitoring of real water sample obtained by the present method was in good agreement with those obtained by the standard reflux titrimetric method. The flowinjection-photodiode chemiluminescence detection systemfor COD determination was lowcost, simple, and robust. In addition, the automation and maintenance of the system were easy.