Carbon-based pseudo-supercapacitors are one of the most promising electrical energy storage devices with high power density and long cycle life that is strongly desired for electric vehicles but their capacitance needs to be improved. Here, a new approach and design principle to enhance the energy density have been developed with the density functional theory methods. The results reveal that compared with pure carbon, the energy density could be enhanced significantly via heteroatom-doping. An intrinsic descriptor is discovered to establish a volcano-shaped relationship that correlates the capacitance with the heteroatom-doping structures of carbon nanomaterials, from which the best electrode structures are identified, which are in good agreement with the experimental results. Th strategies for further enhancing energy densities are proposed for rational design and fabrication of high-energy-density supercapacitors.