Abstract: In order to study the degradation of azo dyes by bio-activated persulfate (PDS) produced by the microbial fuel cell (MFC), methyl orange (MO) was selected as the target pollutant. The effects of pH, PDS concentration, initial MO concentration, and inorganic anions on the degradation of MO were investigated. The results showed that when the initial pH was in the range of 5 to 3, the removal rate of MO increased with the decreasing pH. When the initial pH was lower than 3, the removal rate decreased with the decreasing pH. The removal rate of MO decreased with the increasing initial concentration of MO. When the dosage of PDS was 1-2 mmol/L, the MO removal rate increased with the increasing PDS dosage. When PDS dosage exceeded 2 mmol/L, the removal rate decreased with the increasing PDS dosage. The optimal reaction conditions were as follows:initial pH=3, initial concentration of MO=0.10 mmol/L, dosage of PDS=2 mmol/L. MO removal efficiency reached 86.5% after 4 h. The inorganic anions had inhibitory effect on the removal of MO. When the dosage of each inorganic anion (HCO₃^-, NO₃^-, CO₃^2-) was 10 mmol/L, the removal rates of MO were 64.2%, 68.8% and 76.1%, respectively. But the removal of MO was not significantly affected by Cl^-. Quenching experiments showed that the main free radicals in the system were SO₄^-·and a small amount of·OH. According to the relationship between MO structure and characteristic absorption peaks obtained with UV-Vis spectroscopy, the pathway of MO degradation was inferred. The azo double bond structure of MO chromophore was broken. Then intermediates containing benzene rings were formed, and finally they were mineralized to form CO₂ and H₂O. These results showed that MFC could effectively activate persulfate to produce sulfate radicals. They had better removal and mineralization effects on azo dyes.