Solid-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.