In this work, phytic acid is used as the protonic acid dopant and soft template to synthesize 3D polyaniline (PANI) nanofiber networks. Then, the PANI nanofiber networks are transformed to porous nitrogen and phosphorus co-doped carbon nanofibers (P-NP-CNFs) by the carbonization and chemical activation process. P-NP-CNFs have a high specific surface area of 2586 m² g⁻¹ and a large pore volume of 1.43 cm³ g⁻¹, which are favorable for enhancing the electrochemical performance of electrical double layer capacitors. Moreover, nitrogen and phosphorus doping in the carbon materials can increase the specific capacitance by a pseudocapacitive redox process. At a current density of 1 A g⁻¹, P-NP-CNFs show a large specific capacitance of 280 F g⁻¹ and a high specific capacitance retention of 94% after 10 000 cycles. In particular, phosphorus doping can broaden the electrochemical window to increase the energy density. Therefore, the energy density of symmetric capacitors based on P-NP-CNFs is up to 22.9 W h kg⁻¹ at a power density of 325 W kg⁻¹, demonstrating that P-NP-CNFs are superior electrode materials for electrical double layer capacitors.