Abstract: After the fuel undergoes pyrolysis and combustion in a biomass boiler, the fly ash contains some unburned carbon with abundant pores. However, their low porosities and specific surface areas cannot meet the requirements of the activated carbon for commercial supercapacitors, and the activation method to improve their pore structures is the key to enhance their applicability. After the one-step KOH activation treatment of the unburned carbon with the particle sizes > 0.2 mm, the results showed that the activated carbon has the largest specific area (1982 m²/g) at an impregnation ratio of 3.5∶1, and the specific capacitance reached 207 F/g at the current density of 1 A/g. The above results indicate that unburned carbon-based activated carbon has a good double layer capacitance performance, providing a reference for the high value-added utilization of biomass fly ash.