Abstract: Natural sea mud(BMFC-0), sediment enriched with exogenous urea(BMFC-1) and exogenous lactic acid(BMFC-2) are taken as three types of marine benthonic sediment microbial fuel cells(BMFCs), and the impacts of the exogenous nutrients on the performance of the BMFCs and electrochemical performonce of electrodes have been investigated.Results indicate that urea and lactic acid obviously influence the number of bacteria in the sediment and the electrochemical performance. The BMFC-2 has the largest amount of bacteria(about 1.08×10¹¹ cfu/m²), which is 2.97 and 13.5 times of those in BMFC-1 and BMFC-0, respectively. Tafel curves show that the electrochemical activity in BMFC-2 anode biofilm is higher than those of BMFC-1 and BMFC-0; the electron transfer activities of the anode in the BMFC-1 and BMFC-2 are respectively 1.30 and 1.63 times of that in BMFC-0. In the closed circuit, output voltage of BMFC-2 is the highest, about 520 mV, and that of BMFC-0 is the lowest, about 175 mV; the maximum output power density of BMFC-2 is 96.57 mW·m^-2, 8.83 and 1.87 times of those of BMFC-0(10.94 mW·m^-2) and BMFC-1(51.57 mW·m^-2), respectively. Based on the anodic biofilm capacity property, a novel model is presented to explain the effect of exogenous nutrients on the electrochemical performance of cells; as the number of bateria on the anodic surface increases, electrons produced by metabolizing increases, the biofilm becomes thicker, and the capacitance of biofilms and the electric double-layer capacitance increase.