A study on the measurement of chemical oxygen demand (COD) by UV spectrophotometry method, discusses the research method which combining the design of optical system and electronic technology. The system consists of three parts: optical, circuit, and software. Optical system applies plane diffraction grating and optical slit, which improves the stability of optical signal. Electrical system takes high precision and low quiescent current amplifier OPA2335, improves the conversion accuracy and responding speed of weak light signal. Software system uses chloride and COD standard curve to exclude interference which chloride produces during measuring COD. It is possible to measure COD in high chloride water. Measuring COD standard solution and COD standard solution with chloride ion and measuring actual water sample comparing with HACHDR6000 besides stability experiment which can prove the system can be used to measure COD in high chloride water.