This article expounds the research of the spectrophotometer used in determined the chemical oxygen demand(COD),discusses the research method of the optical system and electronic system.Optical system using symmetrical Czerny Turner system, and use the holographic plane diffraction grating as light splitting element, which improves the spectral energy and spectral resolution.Electronic systems using the high speed high accuracy logarithmic amplifier log114, improves the detecting precision and response speed.Use the COD standard solution calibration Instrument,Compared with HACHDR6000,that proved the COD detection system’s performance and accuracy are stable and reliable.