An mesophilic upflow anaerobic sludge bed (UASB) reactor treating sugar refinery wastewater with an influent COD 20 000 mg·L-1 and HRT 8 h was introduced, and the impact of pH on its performance and hydrogen-producing acetogens and methanogens were investigated. The results showed that the pH in the UASB dropped from 6.8-7.4 to 5.7-6.7 when the influent pH decreased from 6.9 to 5.4, resulting in a reduction in COD removal by 23.3％ and a propionate increase in effluent by 3.9 times. Polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) of sampled sludge indicated that the diversity of hydrogen-producing acetogens in the UASB was remarkably lower than that of the methanogens. The distribution and dominance of hydrogen-producing acetogens had been observably influenced by the dropped pH. The dominance of propionate-oxidizing bacteria (Pelotomaculum), represented by band Eub 19, were significantly reduced in approach acid condition, while acetotrophic methanogens and the dominant hydrogenotrophic methanogens stood steadily. Furthermore, the dominance of some acid-tolerant hydrogenotrophic methanogens such as Methanobacterium ferruginis and uncultured Methanobrevibacter were enhanced observably following the drop in pH. These results suggested that hydrogen-producing acetogens seemed to be more susceptible to pH than methanogens.