In this study, an instrument for circulating BOD analysis by microbial sensor has been developed. A Pseudomonas. sp immobilized by PVA, was employed for sensitive material BOD sensor. The measure period was about 8 minutes. Both relative error and analytical standard deviation are less than 5%. The instrument has the advantages of high autoimmunization and short measure period. The accuracy and stability of the instrument measured up the national standard by testing different type waste waters and national standard samples.