Lignocellulose-based carbon nanosheet has attracted considerable attention for electric double-layer capacitors (EDLCs) applications for its large specific surface area and good chemical stability, as well as abundance and low cost in precursors. However, the high ion-transport resistance severely restricted lignocellulose-based carbon nanosheets from practical applications. Herein, we employed xylan, the second abundant natural macromolecules, as precursor and thiourea as porogen/dopant to fabricate xylan-derived N/S-codoped hierarchical porous carbon nanosheet (HPCNS) with good capacitive energy storage properties. HPCNS was facilely prepared via a molten salt method followed by a successive multi-step activation, which generated large specific surface area (1275.97 m²·g⁻¹) and hierarchical porous structure, thereby providing abundant sites and channels for ion adsorption and transportation. The obtained HPCNS-based electrode showed a specific capacitance of 147.33 F·g⁻¹ at 0.5 A·g⁻¹ in 6.0 M KOH electrolyte, enabling symmetrical EDLCs deliver a long-term cycle stability of 99.85% capacity retention after 10,000 cycles. Density functional theory (DFT) calculations showed that S-doping scarcely enhanced the ion-electron interaction with electrolyte but significantly improved surface wettability and promoted pore formation, which may provide new insights on the influence of S doping on the electrochemical properties of carbon materials. This research may provide a new avenue for the high-value valorization of biomass into electrochemical energy storage devices.