<p>Hard carbon (HC), one of the most prospective commercialized anode for sodium-ion batteries, attracts wide attention owing to its disordered structure and expanded interlayer spacing, which can allow for sodium ions intercalation/deintercalation. However, the poor initial Coulombic efficiency (ICE) of HC has hindered its large-scale application. Among the precursors employed to prepare HC, phenolic resin (PF) shows a high yield carbon content and good reversible capacity. Herein, the relationship between PFs and ICE has been explored using four commercial phenolic resin-based hard carbons with different solid contents. Then, these pure phenolic resins were modified by adding pores-forming agents, cross-linking curing, and ball-milling. We improve these samples’ ICE beyond 86%. The U-HC sample, characterized by the highest solid-content phenolic resin-modified hard carbon, achieves an impressive ICE of 89.84% and delivers a specific discharge capacity of approximately 354.18 mAh g⁻¹ at 35 mA g⁻¹. This study deepens the understanding of phenolic resin-based hard carbon and offers valuable guidance for achieving high ICE in such materials.</p>