Abstract: Introducing redox pseudocapacitance could effectively improve the specific capacitance of carbon-based electrode materials, and is a promising way to overcome the low energy density of carbon-based supercapacitors. An in-situ electrochemical oxidation method was used to electrochemically generate active oxygen-containing functional groups for B, N co-doped carbon nanosheets to significantly increase the pseudocapacitance. Results show that the degree of oxidation, the pseudocapacitance, and the charge transfer rate of the oxidized carbon nanosheets were effectively increased by co-doping with B and N. Compared with the constant potential oxidation method, the cyclic voltammetry oxidation method was more effective in increasing the total oxygen content of the oxidized electrode and to selectively generate electrochemically active quinone groups. The oxidized electrode had a high specific capacitance of 601.5 F g⁻¹ at a current density of 1 A g⁻¹, retaining 74.8% of the original value at 20 A g⁻¹, revealing a favorable rate capability. The oxidized electrode also had excellent cycle stability, retaining 92.6% of the initial capacitance after 8 000 cycles at 5 A g⁻¹.