Abstract: The carbon felt cathodes are subjected to ammonia heat treatment at different temperatures(650 ℃, 800 ℃and 950 ℃) to investigate the electrochemical performances of the cathodes in marine sediment microbial fuel cells(MSMFCs) before and after modification. The capacitance density(172.53 F/cm²) of the cathodes modified at 950 ℃ is 3.03 times of that of the blank group, and its exchange current density 1.53 times of that of the blank group, indicating that its anti-polarization ability is improved by modification. Meanwhile its charge transfer resistance(34.09 Ω) is 0.37 times of that(90.96 Ω) of the blank group, its output power(569 mW/m²) is 1.69 times of(337 mW/m²) that of the blank group. According to the results, the 950 ℃ ammonia gas modified carbon felt cathode has the best oxygen reduction reaction(ORR) rate.