Understanding the dynamic structural reconstruction/transformation of catalysts during electrochemical CO2 reduction reaction (CO2RR) is highly desired for developing more efficient and selective catalysts, yet still lacks in-depth realization. Herein, we study a model system of copper nanowires with various degrees of silver modifications as electrocatalysts for CO2RR. Among them, the Cu68Ag32 nanowire catalyst achieves the highest activity and selectivity toward methane with an extremely high faradaic efficiency of similar to 60%, about 3 times higher than that of primitive Cu nanowires, and even surpasses the most efficient catalysts for producing methane. By using in situ grazing-angle X-ray scattering/diffraction, X-ray absorption spectroscopy, and Raman techniques, we found that the Cu68Ag32 nanowires underwent an irreversible structural reconstruction and well-stabilized chemical state of Cu on the catalyst surface under the working CO2RR conditions, which greatly facilitates the CO2 to methane conversion. Further analysis reveals that the restructuring phenomenon can be ascribed to a reoxidation/reduction-driven atomic interdiffusion between Cu and Ag. This work reveals the first empirical demonstration by deploying comprehensive in situ techniques to track the dynamic structural reconstruction/transformation in a model bimetallic system, which not only establishes a good understanding of the correlation between catalyst surface structure and catalytic selectivity but also provides deep insights into designing more developed electrocatalysts for CO2RR and beyond.