We report phosphorus (P)-doped activated carbon (AC) as a highly active catalyst for the oxygen reduction reaction (ORR) in air-cathode microbial fuel cells (MFCs). P-doped AC was prepared by impregnating AC with H₃PO₄ (1 M) and then heating at 200 °C, 400 °C, 600 °C, 800 °C and 1000 °C, respectively in the presence of nitrogen. The pretreatment of AC increased the power density of MFCs by 75% from 730 ± 23 mW m⁻² (RAW-SP, untreated AC) to 1278 ± 56 mW m⁻² (SP-800, heated at 800 °C). X-ray photoelectron spectroscopy (XPS) analysis confirmed the successful doping of P in all treated samples, and the introduction of oxygen-containing groups contributed to the increase of power densities and ORR performance. Raman spectra analysis indicated that the AC structure was deformed by the doped P atom. According to X-ray diffraction (XRD) and transmission electron microscopy (TEM), the treated samples revealed a higher graphitization degree than the control. The results of electrochemical impedance spectroscopy (EIS) also showed that the introduction of the P heteroatom reduced the total resistance, which contributed to the improved electrochemical performance. However, high temperature could destroy the AC's structure, which led to the decline of BET surface area and micropore volume. Thus, P-doped AC would be a promising electrocatalyst for the ORR in MFCs.