Abstract: The influence of 2-bromoethane sulfonate (BES) on the operational performance, intermediate metabolite-volatile fatty acid (VFA) and the microorganism population structure of the anaerobic baffled reactor (ABR) was investigated, aiming at the acidification problem in anaerobic reactors. The results showed that BES could decrease the processing performance of ABR, and when the BES dosage was 0.27 mmol/L, the removal efficiency of COD_Cr decreased from 95% to 10% correspondingly. The addition of BES increased the acidification risk of ABR with the pH value decreasing to 5.5 and VFA accumulating. The effluent VFA reached to about 15.3 mmol/L with butyric acid, pentanoic acid and hexanoic acid accounting for more than 36%. When BES was added，the relative abundance of methanogens decreased by 25%, and that of hydrogen-producing acetogenic bacteria and homoacetogenic bacteria increased by 9% and 7%, respectively. The PCR-DGGE and Diversity Index also showed that while adding BES, the microorganism population structure in each compartment of ABR was affected significantly and the bacteria population diversity and richness decreased remarkably. Also, the dominant species changed from Firmicutes and Bacterium to Bacteroidetes, Spirochaetes and δ-proteobacteria.