Microbial electrosynthesis (MES) is a novel CO2 utilization technology. Biocatalysts in this process may use electrons obtained from a photovoltaic system to reduce CO2 to chemicals and realize energy conversion from solar energy to chemical energy. The photoelectric material CuO/g-C3N4 was directly introduced into the MES system using mixed culture as biocatalyst in this study. CuO/g-C3N4 can effectively absorb light and presents satisfactory electron and hole separation ability. Photogenerated electrons from CuO/g-C3N4 enhanced the electron transfer rate and reduced cathodic charge transfer resistance. CuO/g-C3N4 mainly improved the electron supply of electroautotrophic microorganisms through direct electron transfer rather than indirect electron transfer via hydrogen. Photogenerated holes can combine electrons from anode and provide extra driving force to improve the MES performance. Furthermore, the CuO/g-C3N4 photocathode also improved the biocatalytic activity by increasing the total amount of biocatalyst and regulating cathodic microbial community composition. Acetate production rate in MES with the CuO/g-C3N4 photocathode was 2.6 times higher than that of the control group. This study provides a new strategy for semiconductor photocathodes to improve the MES performance with mixed culture. Display omitted •CuO/g-C3N4 presents satisfactory electron and hole separation ability.•Photogenerated e− from CuO/g-C3N4 reduced cathodic charge transfer resistance.•CuO/g-C3N4 can provide extra driving force to improve the MES performance.•CuO/g-C3N4 improved the biocatalytic activity and regulated microbial community.•CuO/g-C3N4 mainly improves the electron supply via direct electron transfer.