Enhancing the working current density is a great challenge that limits the practical applications of the CO2 electrochemical reduction. In this study, a facile method was developed to prepare 3D-Ag nanosheet arrays (3D-Ag NA) electrodes for efficient CO2 reduction into CO through in situ oxidation-reduction. The resulting electrodes exhibited high specific surface area and abundant oxygen vacancies. 3D-Ag NA on Ag foil electrode (3D-Ag NA/Ag) demonstrated a significantly enhanced current density of 25.9 mA/cm2 at −1.06 V vs. RHE in an H-cell, surpassing the performance of Ag foil, Ag nanoparticles and Ag2O nanoparticles electrodes. Moreover, 3D-Ag NA on low cost graphite (3D-Ag NA/C) showed a current density of 24.3 mA/cm2 at a remarkble low potential of −0.96 V vs. RHE. It was found that the formation of a dense Ag2O layer play a critical role in the establishing the 3D-Ag NA. Furthermore, CO2-TPD and DFT studies revealed that the 3D-Ag NA exhibited high CO2 adsorption capacity and low CO2 activation energy. This study presents a promising approach for scalable electrode preparation in the electrochemical reduction of CO2 to CO, enabling high current density and selectivity.