<p>Hard carbon (HC) has emerged as the leading anode material for commercialization in sodium-ion batteries. Nonetheless, key hurdles like the low initial Coulombic efficiency, lackluster rate performance, and insufficient cycling stability remain unresolved. This work sheds light on how hydrothermal pre-treatment plays a pivotal role in tailoring structure and improving sodium storage capabilities in biomass-derived hard carbon anodes, while also clarifying the "adsorption–intercalation–pore filling" mechanism for Na⁺ storage. Hydrothermal pre-treatment effectively stabilizes the spherical morphology of HC anodes while introducing additional active sites, including nanopores, and oxygen-based surface functionalities like carbonyl (C=O) groups. The C=O groups can selectively catalyze the favorable salt reduction and inhibit excessive solvent decomposition, promoting the development of a stable, inorganic-rich solid electrolyte interphase. This substantially accelerates Na⁺ transport and storage kinetics, leading to an overall improved sodium storage performance. The optimized sample demonstrates a high reversible capacity (314.29&#xa0;mAh&#xa0;g⁻¹) with a high ICE (89.54%), as well as a superior rate capability and an outstanding cycling performance with 80.12% capacity retention after 300 cycles at 0.5 C. This work offers valuable insights for advancing high-performance HC anodes specifically designed for sodium-ion battery applications.</p>