A on-line monitoring system was developed for the determination of chemical oxygen demand in water based on full-spectrum analysis. In this system, least-square method was used to obtain the transmission equation between absorbance and chemical oxygen demand(COD) value by measuring absorption spectra of water with known COD value, and then the established equation could inverse the COD values of the unknown water samples. For the COD determination of simulated complicated water samples, the instrumental reliability was well validated by comparing the measurement values of the analyzer with that of laboratory results. The monitoring system provided advantages of simplicity, rapidity, high precision, low consumption and environmental benignity, and was demonstrated an ideal alternative to real-time and on-line monitoring of COD in water.