Electrochemical reduction of CO to formate is economically attractive but improving the reaction selectivity and activity remains challenging. Herein, we introduce boron (B) atoms to modify the local electronic structure of bismuth with positive valence sites for boosting conversion of CO into formate with high activity and selectivity in a wide potential window. By combining experimental and computational investigations, our study indicates that B dopant differentiates the proton participations of rate-determining steps in CO reduction and in the competing hydrogen evolution. By comparing the experimental observations with the density functional theory, the dominant mechanistic pathway of B promoted formate generation and the B concentration modulated effects on the catalytic property of Bi are unravelled. This comprehensive study offers deep mechanistic insights into the reaction pathway at an atomic and molecular level and provides an effective strategy for the rational design of highly active and selective electrocatalysts for efficient CO conversion.