AbstractSolid‐electrolyte interphase (SEI) seriously affects battery's cycling life, especially for high‐capacity anode due to excessive electrolyte decomposition from particle fracture. Herein, we report an ultrathin SEI (3–4 nm) induced by Cu⁺‐tailored double electrical layer (EDL) to suppress electrolyte consumption and enhance cycling stability of CuS anode in sodium‐ion batteries. Unique EDL with SO₃CF₃‐Cu complex absorbing on CuS in NaSO₃CF₃/diglyme electrolyte is demonstrated by in situ surface‐enhanced Raman, Cyro‐TEM and theoretical calculation, in which SO₃CF₃‐Cu could be reduced to CuF₂‐rich SEI. Dispersed CuF₂ and F‐containing compound can provide good interfacial contact for formation of ultrathin and stable SEI film to minimize electrolyte consumption and reduce activation energy of Na⁺ transport. As a result, the modified CuS delivers high capacity of 402.8 mAh g⁻¹ after 7000 cycles without capacity decay. The insights of SEI construction pave a way for high‐stability electrode.