Abstract: Spherical hard carbon materials have shown great potential in sodium-ion batteries (SIBs) due to their abundant raw material sources, excellent performance, and structural stability. Starch, as a common hard carbon precursor, possesses a spherical-like structure that can maintain the consistency of the hard carbon framework while controlling costs. However, during high-temperature carbonization, the polysaccharide molecules in starch break down, leading to structural expansion and foaming, which damages its spherical shape. To address this issue, the treatment of starch with phosphorus-containing polymer molecules is proposed. This process promotes cyclization and crosslinking of surface functional groups on the starch, forming a soft carbon coating layer during high-temperature pyrolysis, effectively preventing the foaming phenomenon. As a result, a soft carbon-coated spherical hard carbon material was successfully synthesized. When used as an anode material for SIBs, it exhibited a high reversible capacity of 289 mAh/g, and even at a high current density of 1 A/g, it maintained a reversible capacity of 189 mAh/g. This synthesis strategy offers a new avenue for developing high-performance spherical hard carbon materials.