Abstract: Petroleum coke (PC) is a valuable precursor for sodium-ion battery (SIB) anodes due to its high carbon content and low cost. The regulation of the microcrystalline state and pore structure of the easily-graphitized PC-based carbon is crucial for creating abundant Na⁺ storage sites. Here we used a precursor transformation strategy to increase the carbon interlayer spacing and generate abundant closed pores in PC-based carbon, significantly increasing its Na⁺ storage capacity in the plateau region. This was achieved by introducing a large number of oxygen functional groups through mixed acid treatment and then using high-temperature carbonization to decompose the oxygen functional groups and rearrange the carbon microcrystallites, resulting in a transition from open to closed pores. The optimized samples provide a large reversible capacity of 356.0 mAh g⁻¹ at 0.02 A g⁻¹, of which approximately 93% is below 1.0 V. Galvanostatic intermittent titration (GITT) and in-situ X-ray diffraction (XRD) analysis indicate that the sodium storage capacity in the low voltage plateau region involves a joint contribution of interlayer insertion and closed pore filling processes. This study presents a comprehensive method for the development of high-performance carbon anodes using low-cost and highly aromatic precursors.