The electrocatalytic CO₂ reduction technology is expected to simultaneously alleviate increasing CO₂ emissions and the depletion of fossil resources. However, it is still a big challenge to improve the selectivity toward valuable multi-carbon products in electrocatalytic CO₂ reduction reaction. In this study, the synergistic role of Cu⁺ and Cu⁰ species in Cu₂O-Cu interfaces is unravelled through density functional theory (DFT) calculations, in which the electrode surface exhibits low free energy of *COCO intermediate formation and H₂O dissociation, which are beneficial to the high selectivity towards multi-carbon products, especially C₂H₄. Guided by these DFT results, an oxide-derived copper electrode activation strategy that builds the synergistic Cu⁺ and Cu⁰ on Cu₂O-Cu interfaces is designed to boost the selectivity toward multi-carbon products. Interestingly, Cu₂O cubes chosen as the pristine catalyst are activated via a square-wave (SW) potential treatment to form SW-Cu₂O cubes that bear Cu⁺ and Cu⁰ species. The as-prepared SW-Cu₂O cubes exhibit superior Faradaic efficiencies for C₂H₄ (60%) and C₂₊ products (75%) in an H-type cell, which are about 1.5 times that of the Cu₂O cubes. This study demonstrates the synergistic Cu⁰ and Cu⁺ on Cu₂O-Cu interfaces for improving the selectivity of a specific valuable multi-carbon product in electrocatalytic CO₂ conversion.