A nitrogen and phosphorus co-doped carbon sphere was synthesized and prepared by a two-step hydrothermal activation pyrolysis method using agricultural materials, in which forestry waste walnut shells, urea, and phosphoric acid were used as carbon, nitrogen, and phosphorus sources, respectively, for the efficient treatment of heavy metals Cr(VI) in wastewater. On this basis, a supercapacitor with high capacitive performance was investigated. The adsorption capacity of the N, P co-doped carbon sphere (N2PC) was optimal for Cr(VI), and the abundant functional groups on the surface of the carbon spheres significantly promoted the adsorption and reduction of Cr(VI). The adsorption capacity of the carbon material was up to 100.55 mg/g at 318 K, and the pseudo-second-order kinetic model and the Langmuir adsorption isotherm model were used to describe the adsorption process. Before and after adsorption, the material was applied to the electrode material of the supercapacitor, and the capacitive performance of the adsorbed material was satisfactory as well as with excellent cycling stability which 93% capacity retention even after 5000 cycles.