The reductive decolorization of azo dye wastewater was investigated by using a dual_chambered bioelectrochemical cell equipped with different cathode materials (graphite and rutile_coated graphite). The experimental data show the feasibility of electricity generation by utilizing the model azo dye of methyl orange (MO) as the cathodic electron acceptor along with the color removal. Compared with MO reduction in traditional microbial fuel cell (MFC), an increase of the efficiency for MO reduction as well as the electricity production was successfully achieved in the irradiated rutile_cathode system. Quantification of the internal resistance by fitting the electrochemical impedance spectra (EIS) data to an equivalent circuit showed that the polarization resistance (Rp) of rutile_cathode significantly decreased from 1378 Ω (dark control) to 443.4 (light control) Ω. The results demonstrate that the cathodic electron transfer process in the irradiated rutile_cathode system is driven by the photocatalysis of rutile. The photoreduction of MO at different initial concentrations obeys the pseudo_first_order kinetics, and the reaction constant increases with the decrease of the initial concentration of the dye. Based on an analysis of decolorization products, the authors put forward a possible cathodic reaction mechanism for the photoreductive decolorization of MO, i.e., the azo bond of MO is cleaved by photoelectrons at the irradiated rutile_cathode, resulting in the formation of colorless hydrazine derivative.