Cu-based catalysts selectively convert CO2/CO into valuable C2+ oxygenates and hydrocarbons electrochemically, which is regarded as a promising strategy for carbon cycle utilization. Herein, we synthesized CuxP2Ox+5 (x=2, 4, 5) by introducing phosphorous in cupric oxide, which is in situ electrochemically reconstructed into metallic Cu with highly porous structure during CO electrolysis. Physicochemical characterizations demonstrate various degree of grain boundary generation, which depends on Cu atom density in CuxP2Ox+5 cell volume. The reconstructed CuxP2Ox+5 shows a grain boundary-dependent performance in CO electrolysis, with C2+ Faradaic efficiency over 90% at current density greater than 1.0 A cm–2. Among them, the reconstructed Cu5P2O10, with the highest surface density of grain boundary, achieves a C2+ current density of 1.70 A cm–2 and a C2+ formation rate of 575.8 μmol min–1. Operando Raman spectra reveal a strong CO adsorption with dominant configurations of atop and bridge. Density functional theory calculation indicates that grain boundary provides active C−C coupling and H2O dissociation sites, which facilitate *CO−COH formation for C2+ production.