Electrochemical　CO2　reduction　(CO2RR)　is　a　promising　technology　to　mitigate　the　greenhouse　effect　and　convert　CO2　to　value-added　chemicals.　Yet,　achieving　high　catalytic　activity,　selectivity,　and　stability　for　target　products　is　still　a　big　challenge.　Herein,　interstitially　Sn-doped　Bi　(Sn-x-Bi,　x　is　the　atomic　ratio　of　Sn　to　Bi,　x　=　1/2,　1/16,　1/24　or　1/40)　nanowire　bundles　(NBs)　are　prepared　by　reducing　Sn-doped　Bi2S3.　Notably,　Sn-1/24-Bi　NBs　exhibit　ultrahigh　formate　selectivity　over　a　broad　potential　window　of　1400　mV　(Faradaic　efficiency　over　90%　from　-0.5　to　-1.9　V　vs.　reversible　hydrogen　electrode　(RHE))　with　an　industry-compatible　current　density　of　-319　mA　cm(-2)　at　-1.9　V　vs.　RHE.　Moreover,　superior　long-term　stability　for　more　than　84　h　at　similar　to-200　mA　cm(-2)　is　realized.　Experimental　results　and　density　functional　theory　(DFT)　calculations　reveal　that　interstitially　doped　Sn　optimizes　the　adsorption　affinity　of　*OCHO　intermediate　and　reduces　the　electron　transfer　energy　barrier　of　bismuth　catalyst,　resulting　in　the　remarkable　CO2RR　performance.　This　study　provides　valuable　inspiration　for　the　design　of　doped　electrocatalysts　with　enhanced　catalytic　activity,　selectivity,　and　durability　for　electrochemical　CO2-to-formate　conversion.