A surface strategy is introduced to enhance ethylene selectivity in the electrochemical reduction of CO₂. By modifying CuO with dodecanethiol (DDT), the Faradaic efficiency (FE) for ethylene reaches 79.5%, significantly higher than unmodified CuO. In situ investigations reveal that DDT facilitates CO₂ transport, enhances *CO coverage on the catalyst surface, and stabilizes the Cu(100) facet. Density functional theory (DFT) calculations support that the thiol-stabilized Cu(100) reduces the activation energy barrier for the asymmetric C–C coupling between *CO and *CHO intermediates, enhancing ethylene selectivity. The study provides insights into the mechanism of CO₂ to ethylene conversion and offers a strategy for designing Cu-based catalysts with high selectivity. The DDT-modified CuO electrode demonstrates improved stability and activity, with ethylene FE remaining high even after prolonged operation. The results highlight the importance of Cu(100) facets in promoting ethylene selectivity and demonstrate the potential of surface modification in enhancing CO₂ electroreduction efficiency. The findings contribute to the development of more efficient catalysts for the selective conversion of CO₂ into valuable chemicals and fuels.A surface strategy is introduced to enhance ethylene selectivity in the electrochemical reduction of CO₂. By modifying CuO with dodecanethiol (DDT), the Faradaic efficiency (FE) for ethylene reaches 79.5%, significantly higher than unmodified CuO. In situ investigations reveal that DDT facilitates CO₂ transport, enhances *CO coverage on the catalyst surface, and stabilizes the Cu(100) facet. Density functional theory (DFT) calculations support that the thiol-stabilized Cu(100) reduces the activation energy barrier for the asymmetric C–C coupling between *CO and *CHO intermediates, enhancing ethylene selectivity. The study provides insights into the mechanism of CO₂ to ethylene conversion and offers a strategy for designing Cu-based catalysts with high selectivity. The DDT-modified CuO electrode demonstrates improved stability and activity, with ethylene FE remaining high even after prolonged operation. The results highlight the importance of Cu(100) facets in promoting ethylene selectivity and demonstrate the potential of surface modification in enhancing CO₂ electroreduction efficiency. The findings contribute to the development of more efficient catalysts for the selective conversion of CO₂ into valuable chemicals and fuels.