The efficient conversion of renewable biomass to advanced carbons with high porosity and heteroatoms doping is of huge significance towards energy-related applications. However, most of the resultant carbon materials are always bulk materials with low aspect ratio and unsatisfactory electronic properties owing to the severe activation process. In this work, the KNO3 modified molten-salt (KCl/LiCl) strategy is explored to fabricate two dimensional nitrogen-doped carbon nanosheets (NCNs) by using agaric as the precursor. Benefiting from its large surface area (1376 m(2) g(-1)), high-ratio micro-porosity (72%), high-content N doping (4 at.%) and high graphitization, the optimized NCNs sample is endowed with exceptional electrochemical capacitances both in aqueous electrolyte (6 M KOH) and organic electrolyte (1 M tetraethylammonium tetrafluoroborate/acetonitrile), when evaluated as a competitive electrode for supercapacitors. It is strongly envisioned that the method developed here holds great potential for advanced porous heteroatom-doped carbons from natural biomass for next-generation supercapacitors.