Electroreduction derived Bi-based materials have been recognized as promising catalysts for CO₂ reduction to formate, however, it remains unclear whether the reconstructed Bi vacancies significantly influence the reaction performance. Herein, we first construct vacancies-rich Bi nanosheets through an in situ electroreduction process, in which positron annihilation lifetime spectroscopy unravels the presence of Bi monovacancy states, confirmed by in situ Raman spectra. In situ attenuated total reflection-surface enhanced infrared absorption spectroscopy and operando Raman spectra unravel the Bi vacancies could enhance the adsorption of *OCHO key intermediates and facilitate their reaction kinetics. Moreover, density-functional-theory calculations demonstrate the Bi vacancies induce electron aggregation, thereby stabilizing the *OCHO intermediates. As a result, the Bi nanosheets with abundant vacancies achieve a remarkable formate Faradaic efficiency of 97.1% at current density of 200 mA·cm⁻², and produce 1 M pure formic acid solution for 250 hours in a solid electrolyte reactor.