Biomass-derived O- and N-doped porous carbon has become the most competitive supercapacitor electrode material because of its renewability and sustainability. We herein presented a facile approach to prepare O/N-doped porous carbon with cotton as the starting material. Absorbent cotton immersed in diammonium hydrogen phosphate (DAP) was activated at 800 °C (CDAP800s) and then was oxidized in a temperature range of 300-400 °C. The electrochemical capacitance of the impregnated cotton was significantly improved by doping with O and N, and the yield was improved from 13% to 38%. The sample oxidation at 350 °C (CDAP800-350) demonstrated superior electrical properties. CDAP800-350 showed the highest BET surface area (1022 m 2 g −1 ) and a relatively high pore volume (0.53 cm 3 g −1 ). In a three-electrode system, the CDAP800-350 electrodes had high specific capacitances of 292 F g −1 in 6 M KOH electrolyte at a current density of 0.5 A g −1 . In the two-electrode system, CDAP800-350 electrode displayed a specific capacitance of 270 F g −1 at 0.5 A g −1 and 212 F g −1 at 10 A in KOH electrolyte. In addition, the CDAP800-350-based symmetric supercapacitor achieved a high stability with 87% of capacitance retained after 5000 cycles at 5 A g −1 , as well as a high volumetric energy density (18 W h kg −1 at 250 W kg −1 ). Biomass-derived O- and N-doped porous carbon has become one of the most competitive supercapacitor electrode material because of its renewability and sustainability.