In order to make up for the deficiency of traditional method of 5 d for dilution and seeding method (BOD5), the on line biochemical oxygen demand(BOD) measuring instrument based on microbial sensor was designed. BOD biosensor was the combination of dissolved oxygen electrode and microbial film. Measurement of BOD using microbial membrane sensor was a fast and effective way for measuring biodegradable organic compounds in water. The principle of BOD biosensor was discussed. Whole structure of on-line BOD measuring instrument was given. The continuous sampling system and sensor signal process circuit were designed. The methods of automatic calibration, temperature control, and data process and concentration calculation of organic compounds were expounded. The relative errors of designed BOD on-line instrument with standard samples were less than 5%, and had a better correlation with the method of dilution and seeding. The designed instrument could be used for rapid determination and evaluation of biodegradable organic compounds in wastewater.