Abstract: Porous carbons are widely used in supercapacitors, owing to their long cycle life and natural abundance. However, most of these electrode materials give a low capacitance, which leads to low energy density. Cu-doped biomass-derived activated carbons (Cu-ACs) were synthesized using a simple, low-cost carbonization and KOH activation method. The copper nanoparticles had mixed valence states (CuO, Cu₂O, Cu⁰) and were uniformly dispersed on the surface of the AC. Due to the fast electron/ion transfer paths provided by the pore structure, and an accelerated redox reaction between the three Cu species, the Cu-ACs achieved an excellent capacitive performance. In a three-electrode system, the Cu-AC sample prepared by KOH activation with a KOH/ （Cu+char）mass ratio of 2 had a high specific capacitance of 360 F g⁻¹ at 0.5 A g⁻¹, 1.21 times that of AC (163 F g⁻¹). When it was fabricated into a symmetric capacitor, the device had a good electrochemical performance with a specific capacitance of 143.44 F g⁻¹ at 0.5 A g⁻¹ and a good cyclic stability with an 81.8% capacitance retention after 6000 cycles.