Abstract: Supercapacitors have been attracted tremendous attention due to their high power density and long cycle life, etc. The electrode material is the main factor affecting electrochemical properties. Graphene/manganese dioxide composites (RGO/MnO₂) were prepared using one pot hydrothermal method with graphene oxide (GO) as carbon source, as well as H₂O₂ and KMnO₄ as MnO₂ precursors. It was found that sphere-like MnO₂ distributes on the graphene sheets by the microstructure tests. The energy storage mechanism of the composite was discussed. It displays that the reaction is the surface dominant process. The surface capacitance accounts for 86.2% of the total capacitance at 5 mV·s⁻¹, While it can account for 97.3% at 200 mV·s⁻¹. In order to assemble a device with high energy density, this work fabricated an asymmetric supercapacitor (ASC, RGO/MnO₂//RGO) using the RGO/MnO₂ as the positive electrode and RGO as the negative electrode, respectively, which exhibits high energy density (72.8 W·h·kg⁻¹ at 100 W·kg⁻¹).