MnO2 is regarded as an ideal material of supercapacitor since its low-cost, environment friendly and high specific capacitance but hindered by its poor electrical conductivity. Developing a composite electrode that combines nano-structure MnO2 with a conductive skeleton such as carbon materials could make up for the shortcomings. Here, porous activated carbon (PAC) is synthesized by using low-cost wheat bran as biomass carbon precursor and a mixture of NaCl/ZnCl2 as combined solvent-porogen. The resultant PAC sample presents a hierarchical porous structure and large specific surface area up to 1058 m2 g−1. Afterwards, MnO2 nanosheets decorated PAC (MnO2@PAC) is prepared via an in-situ hydrothermal deposition. It is a key finding that the ion/electron transfer kinetics of MnO2@PAC could be effectively improved by the addition of hierarchical porous carbon. Thus, the MnO2@PAC electrode displays a high specific capacitance (258 F g−1 at 1 A g−1) and superior rate performance (82.8% capacitance retention with the current density ranging from 1 A g−1 to 20 A g−1). Furthermore, an asymmetric supercapacitor is assembled by employing the MnO2@PAC as the positive electrode and PAC as negative electrode, which exhibits a high energy density of 32.6 Wh kg−1 and as well as 93.6% capacity retention at over 10,000 charge/discharge cycles.