Highly porous carbon (3047 m2 g−1) with a graphene nanoplatelet microstructure is obtained by using a biomass waste as a new carbon source. Introducing high-energy ball milling treatment during the synthesis procedure can greatly improve the surface wettability of carbon and ensure the homogenously contact between carbon and KOH, thus leading to high reactivity of the KOH activation. The highly efficient activation endows this carbon with favorable features for supercapacitors, such as high surface area with coexistence of rich micropores and mesopores, unique graphene nanoplatelet microstructure with good conductivity and many capacitive active sites. The as-prepared carbon exhibits high specific capacitances of 329 and 311 F g−1 in 2 m KOH and 1 m H2SO4 electrolyte at 1.0 A g−1, respectively. In 1 m TEABF4/AN electrolyte, a high working voltage of 3 V and excellent rate performance can be obtained based on a two-electrode full cell. The energy density of the carbon/carbon symmetric capacitor reaches 49.5 W h kg−1 and the extreme power density can reach 10.8 kW kg−1. The results suggest this biomass-waste-derived carbon can serve as a low-cost and high-performance electrode material for supercapacitors with universal electrolytes.