Abstract: [Objective] To investigate the accumulation mechanism of volatile fatty acids (VFAs) the shift of bacterial community and the contribution of syntroph acetogenic bacteria during the anaerobic fermentation of sewage sludge under different pH values. [Methods] The VFAs were determined at different pH conditions during the sludge anaerobic fermentation process. The Terminal Restriction Fragment Length Polymorph (T-RFLP) and Fluorescence In Situ Hybridization (FISH) were used to explore the variation of bacterial community and the quantity of syntroph acetogenic bacteria, respectively. [Results] When the pH was controlled at 10.0, yields of VFAs and acetate were 652.6 mg COD/g-VS and 322 mg COD/g-VS,respectively (COD, Chemical Oxygen Demand; Volatile Solid, VS). They are much higher than that under neutral and acid pH conditions. The results by T-RFLP showed that Granulicatella was the dominant bacterial population at pH 12.0, while Peptostreptococcus dominated at pH 10.0. When the pH was adjusted from 7.0 to 3.0, the dominant bacterial populations were Clostridium and Bacillus, respectively. Based on the FISH result, the amount of syntrophic acetogenic bacteria at neutral condition was higher than that under the acid and alkali conditions. When the pH value was 10.0, the relative abundance was below 0.01% in total microorganism. [Conclusions] pH not only altered the yields of VFAs and acetate, but also changed the bacterial community structure. Under the alkali condition, the VFAs accumulation was mainly from the hydrolysis fermenting bacteria. At the neutral and acid conditions, the syntroph acetogenic bacteria made more contribution for the acetate accumulation.