The electricity-driven bioreduction of carbon dioxide to multi-carbon organic compounds, particularly acetate, has been achieved in microbial electrosynthesis (MES). MES performance can be limited by the amount of cathode surface area available for biofilm formation and slow substrate mass transfer. Here, a fluidized three-dimensional electrode, containing granular activated carbon (GAC) particles, was constructed via MES. The volumetric acetate production rate increased by 2.8 times through MES with 16 g L-1 GAC (0.14 g L-1 d(-1)) compared with that of the control (no GAC), and the final acetate concentration reached 3.92 g L-1 within 24 days. Electrochemical, scanning electron microscopy, and microbial community analyses suggested that GAC might improve the performance of MES by accelerating direct and indirect (via H-2) electron transfer because GAC could provide a high electrode surface and a favorable mass transport. This study attempted to improve the efficiency of MES and presented promising opportunities for MES scale-up.