Because of the special lamellar structure, numerous pores, and large effective specific surface area, the two-dimensional porous carbon material is very beneficial for the storage and transmission of energy. Herein, a facile method for preparing two-dimensional porous carbon nanosheets from sodium lignosulfonate with excellent electrochemical performance is reported. In this study, porous carbon nanosheets (PCNs) were obtained by the carbonization of the material, in which sodium lignosulfonate was coated on the boric acid surface, followed by activation by potassium hydroxide. By changing the thickness of the lamella structure and the subsequent activation process, the hierarchical pore structure was prepared in an effectively controlled manner. When the ratio of boric acid to sodium lignosulfonate was 4 : 1, the material possessed an ideal architecture and the best electrochemical performance. Porous carbon nanosheets with reasonable pore size distribution could increase the electron transfer rate between the electrolyte and the electrode material. The capacitance of PCN-4 was found to be 198 F g⁻¹ at 1 A g⁻¹ and could still maintain about 90% when the current density was increased to 20 A g⁻¹. In addition, excellent cycling stability was obtained, and the capacitance retention was more than 91% after 5000 cycles under 10 A g⁻¹. Thus, this research discovered a facile way to utilize sodium lignosulfonate, which is used for biomass conversion and renewable energy utilization.