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 (Snx-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, Sn1/24-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 ∼−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.