The porous carbon with angstrom-sized pores is important in supercapacitor applications, because of its high pack density and high specific capacitance. In this paper, a facile method is proposed for the preparation of hierarchical porous carbon with high-volume angstrom-sized pores. Onion, as the typical biomass in this research, is used as carbon precursor. First, onion was etched by KOH to obtain a water-soluble lignin-potassium-salt/cellulose composite. This composite was further pyrolyzed under N atmosphere to obtain onion derived porous carbon (OPC). The morphology and porous structure of OPC were characterized by scanning electron microscope and N adsorption/desorption. The supercapacitive performances of OPC were investigated by cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge-discharge. OPC shows high specific surface area with high-volume angstrom-sized pores in carbon matrix. When used as supercapacitor electrode materials, OPC shows high specific capacitance and good cycling stability. This paper opens a general way to prepare porous carbon from biomasses, which will promote the development of biomass utilization, preparation of porous carbon and supercapacitors.