In this article, Fe3O4/actlvated biocarbon (KWB) composites (KBFe) have been prepared via a chemical coprecipitation method. First, the KWB was derived from biomass waste, sugarcane bagasse, and then, the KBFe composites were obtained by using KWB as the scaffolds for Fe3O4 growth. Results indicated that the chemical composition and microstructure of the KBFe were intimately related to the mass ratio of Fe3O4 and KWB, which impacted the electrochemical performance of the KBFe profoundly. Especially, the KBFe-4 (the mass ratio of Fe3O4/KWB is 1) displayed the highest specific capacitance of 342 Fg(-1) at a current density of 1 Ag-1. Furthermore, the asymmetric two-electrode system based on MnO2/KWB composite (KWBM) as positive electrode and KBFe-4 as negative electrode was successfully assembled with a voltage window of 0-1.6 V, and exhibited high energy density (29.1 Wh kg(-1)) and power density (800 W kg(-1)), along with excellent cycling stability (89.5% capacitance retention after 1000 cycles).