The aim of this study is to investigate the effect of sulfate-reducing bacteria on performance of a mixed culture microbial electrosynthesis system (MES). The two-chamber MESs were operated under different cathode potentials (-0.5, -0.6, -0.7, and -0.8V) with or without addition of 6mM sulfate. At -0.7V, acetate production and electrons harvesting in the MES with the sulfate addition were 31.81mM and 5152C, respectively, which improved by 2.7 and 2.4times compared to that without sulfate. With sulfate, the biomass, proportion of live cells, and electrochemical activity of cathode biofilm were enhanced at all the potentials. At -0.7V, the relative abundance of Acetobacterium and Desulfovibrionaceae was 14.2% and 36.7% with sulfate, respectively, compared to 17.4% and 7.3% without sulfate. At -0.7 and -0.8V, the sulfate-reducing bacteria can promote the electron transfer of cathode biofilm and enhance the acetate production.