A type of unusual interconnected graphitized carbon nanosheets (GCNS) was fabricated from biomass waste, i.e., inner shaddock skins using a facile combined method of simultaneous carbonization-activation and post-vacuum-annealing processes. The obtained GCNS has an optimized integration of a cage-like high-aspect-ratio nanosheet structure (∼8 nm thickness), a large surface area of 2327 m2 g−1 and hierarchical meso/micropore systems (82.3% mesopore volume). Given the effect of post-vacuum-annealing process, enhanced graphitization degree and excellent electronic conductivity (7.9 S cm−1) were obtained for the GCNS. The enhanced graphitization degree not only effectively improves electronic/ionic-transport kinetics in favor of rate capability but also prevents electrolyte degradation thus benefitting cyclic life. The ionic liquid-based supercapacitors assembled with symmetric GCNS electrodes exhibited an ultrahigh rate capability of 87% at current density of 100 A g−1 (holding 132 F g−1) and a long cyclic life of 97.6% capacitance retention after 10,000 cycles. The excellent rate capability resulted in an integrated high energy-power property at an energy density of 56 Wh kg−1 (29.2 Wh L−1), corresponding to a power density of 93 kW kg−1 (48.4 kW L−1).