The electrochemical conversion of CO2 into hydrocarbons is an important approach to store sustainable energy and address climate concerns. However, it is a huge challenge to unearth a promising model for elucidating the role of dopants and vacancies on catalysts upon CO2 electroreduction. Herein, porous indium oxynitride nanosheets with simultaneous incorporation of nitrogen dopant and oxygen vacancy (Vo-N-InON) are reported for achieving efficient CO2 conversion to formic acid (HCOOH). As a result, the catalyst exhibits an extremely high formate selectivity of 95.1% at a low potential of −0.8 V versus reversible hydrogen electrode (RHE) compared with pristine In2O3, Vo-In2O3, and InN, delivering a large partial current density of 121.1 mA cm–2 for formate production at −1.13 V versus RHE in the flow cell. Density functional theory calculations reveal that the generation of *OCHO intermediate is the rate-determining step. The synergistic effect between nitrogen dopants and oxygen vacancies contributes to the activation of CO2, facilitates the charge transfer, and reduces the reaction free energy of *OCHO protonation. This work not only discloses a fundamental understanding of synergistic effects between nitrogen dopants and oxygen vacancies to improve catalytic performance, but also provides an effective platform toward CO2 conversion.