Although the operating mechanism of sodium-ion battery (SIB) resembles that of lithium-ion battery, common film-forming additive for lithium-ion battery does not play its role in SIB. Therefore, it is essential to tailor new additives for SIB. Hard carbon (HC), as the most used anode material of SIB, has the disadvantage of interphasial instability, especially under the condition of long-term cycling. The incessant accumulation of electrolyte decomposition products leads to a significant increase in interphasial impedance and a sharp decline in discharge capacity. In this work, N-phenyl-bis(trifluoromethanesulfonimide) (PTFSI) was proposed as a novel film-forming electrolyte additive, which effectively enhances the long-term cycling performance for HC anode in SIB. The passivation film generated from the preferential reduction of PTFSI improves the capacity retention of HC/Na half-cell from 0% to 68% after 500 cycles. Profoundly, the enhanced interphasial stability of HC anode results in a 52% increase in capacity retention of HC/Na₃V₂(PO₄)₃ full-cells after 100 cycles.