The surface of activated carbon was modified by melted sodium sulfide and its specific capacitance was evaluated by the cyclic voltammetry experimental data. The reason for specific capacitance changed with this treatment and the mechanism of energy storage were investigated by FT-IR, BET, EIS, and electrophoresis experiments. The results showed that the specific capacitance of activated carbon in 10% sodium sulfate electrolyte solution was elevated from 44.6 F/g to 80.8 F/g after treated with sodium sulfide at 1000℃ for 1 h. The increment of specific capacitance mainly came from double-layer capacitance and secondarily from redox pseudocapacitance (the ratio of pseudocapacitance to total capacitance was only 5.6%). After adding 37 mmol/L ferric chlorides in the electrolyte solution, the specific capacitance was further elevated to 103 F/g, in which the proportion of pseudocapacitance capacitance to total capacitance amounted to 12.6%. However, the capacitance value was unstable and decreased gradually during subsequent cycles, which may be caused by the electronegativity of the activated carbon surface. It had been verified by the electrophoresis experiment that the surface of activated carbon after heat treatment by sodium sulfide was positively charged. The law of same charge mutual repulsion lead to unstable adsorption of Fe3+ on the surface of activated carbon. However, if 30 mmol/L K3[Fe(CN)6] was added in the electrolyte solution, the specific capacitance was promoted to 126 F/g and kept stable, even slightly and slowly increased during subsequent cycles. The cyclic voltammetry curve showed that the specific capacitance at this time still mainly came from the double-layer and the EIS experiment confirms that the charge exchange resistance was dramatically reduced when the [Fe(CN)6]3– anion appeared in the electrolyte solution. According to the shape of the CV curve and the results of EIS, FT-IR and BET, it can be concluded that the energy-storage mechanism of activated carbon changed from physical double-layer to physical and chemical double-layer together with redox pseudocapacitance after treated by sodium sulfide.