In this paper, MnO2/allium-giganteum-like biocarbon (KWB) composites (KWBM) have been prepared by a simple method. First, the biocarbon was derived from sugarcane bagasse by one-step carbonization and activation method, and then, MnO2 nanosheets anchored on the surface of biocarbon and the KWBM was obtained. The analysis results demonstrated that chemical composition and microstructure of KWBM were closely related to the mass ratio of KMnO4/KWB, and which further impacted electrochemical performance of KWBM profoundly. Especially, the KWBM-4 (the mass ratio of KMnO4/KWB is 2) exhibited a highest specific capacitance of 402 Fg(-1) at a current density of 1 A g(-1) in three-electrode system. The asymmetric two-electrode system with outstanding energy density was assembled by employing the KWBM-4 as the positive electrode and the KWB as the negative electrode. The two-electrode system displays a high energy density of 25.9 Wh kg(-1) at a power density of 750 W kg(-1) within a potential rage of 0-1.5 V. Furthermore, the system exhibited high cycle stability with only 5.8% loss of its initial capacitance after 2000 cycles. (C) 2017 Elsevier B.V. All rights reserved.