A device to assay the chemical oxygen demand catalyzed by photoelectricity belongs to the environment analysis chemical sensor. The device is the modified product of the traditional water chemical oxygen demand analyzer catalyzed by electricity. The modified part is: the analyzing detecting device is installed with the ultraviolet light resource and the quartz window; the working electrode is themetal oxide modified electrode with the photoelectricity co-catalyzing activity; the working and the assistant electrodes are adhere to the two sides of the lower surface center on upper cover board.It can degrade the water organic matter efficiently and get the water chemical oxygen demand quickly and accurately.