Electrocatalytic CO₂ reduction reaction (ECO₂RR) converts CO₂ to high-value chemical products and promotes the carbon cycle. Sulfur (S)-modified copper (Cu) and bismuth (Bi)-based catalysts have been recognized as promising catalysts for ECO₂RR. Both of them are highly active for selective formate generation, however, their poor stability and severe competing hydrogen evolution reaction (HER) remain challenging. Herein, S-doped Cu coated with Bi (Bi/Cu-S) is developed to improve ECO₂RR selectivity to formate. Bi/Cu-S/brass mesh (BM) electrode material for ECO₂RR was prepared by electrodepositing Bi on the surface of Cu-S/BM nanowires obtained from CuS/BM after the electroreduction. The Faradaic efficiency (FE) of the formate reaches the maximum of 94.3% at −0.9 V vs. reversible hydrogen electrode (RHE) with a partial current density as high as −50.7 mA·cm⁻² and a yield of 30.7 mmol·h⁻¹·cm⁻² under 0.5 M KHCO₃ electrolyte. Meanwhile, the FE of formate is higher than 90% in the voltage range of −0.8 to −1.0 V vs. RHE. It also shows good stability at −0.9 V vs. RHE with the FE of formate remaining above 93% after a 10 h reaction. Density functional theory (DFT) calculations demonstrate that the Bi/Cu-S structure promotes the adsorption of CO₂ and effectively inhibits HER by enhancing the adsorption of *H to a great extent, improving the selective conversion of CO₂ to formate. This work deepens the understanding of the mechanism of Cu-Bi-based catalysts and S-modified Cu-based catalysts in selective ECO₂RR to formate, and also provides a new strategy for catalyst design.