An oxidized copper species (Cu delta+) on the metallic copper surface is critical to the activity and selectivity of electrochemical reduction of CO2 gas. However, Cu delta+ species are easily reduced under working conditions of CO2 electroreduction. Herein, we propose an interface engineering strategy to stabilize Cu delta+ species; specifically, ZnOx nanoparticles are grown on a copper foil to generate a Cu/ZnOx interface. The interface stabilizes the surface Cu2+ species and delivers high methane selectivity (similar to 36%) and long-term durability (>12 h) at a potential of -1.1 V versus reversible hydrogen electrode (RHE) for CO2 reduction. By combining comprehensive characterizations with simulation experiments, we identify cupric species as active sites for CH4 formation, which is confirmed by density functional theory calculations. Our work demonstrates that interface engineering is a promising way to stabilize active sites and boost selective CO2 electroreduction.