Abstract: Transforming waste resources into energy storage materials is a new way to convert them into value-added products and help solve the problems of energy shortage and environmental pollution. A nitrogen-phosphorus co-doped activated carbon was synthesized from waste cotton fabric by combining carbonization and activation in ammonium polyphosphate and a molten salt system (ZnCl₂ and KCl with a molar ratio of 52∶48). The morphology, microstructure and composition of the activated carbon were characterized by SEM, nitrogen adsorption, Raman spectroscopy and XPS. Cyclic voltammetry and galvanostatic charge/discharge were used to test the supercapacitor performance of the activated carbon. Results show that the co-doped activated carbon had a specific surface area of 751 m²·g⁻¹, a specific capacitance of 423 F·g⁻¹ at a current density of 0.25 A·g⁻¹, and a capacitance retention rate of 88.9% after 5 000 cycles at a current density of 5 A·g⁻¹. The energy density was 28.67 Wh·kg⁻¹ at a power density of 200 W·kg⁻¹ for a symmetrical supercapacitor using the activated carbon.