The goal of this study is to make a high-performance cathode in a simple way, so that it can better serve marine sediment microbial fuel cells. We design a cathode by combining graphene with carbon fiber through a binder, and apply it for the first time to these cells, to improve the electrochemical performance of these cells while increasing the specific surface area of the cathode. This cathode shows excellent performance in a laboratory simulated marine environment. The specific surface area of graphene modified polyacrylonitrile fiber cathode reaches 41.13 m2/g, 1.5 times larger than that of the blank cathode (26.98 m2/g). Its capacitance and exchange current density are 6.1 times and 16.4 times higher than that of theblank cathode, respectively. The anti-polarization ability of the modified cathode was significantly improved, and the maximum power density of the cell equipped with the G/P cathode (2.12 kW/kg) is 6.24 times that of the blank (0.34 kW/kg). In terms of stability, it is quite resistant to prolonged seawater immersion or scouring. This excellent mechanical stability can extend the cells’ life and reduce replacement costs.