Copper electrocatalysts have been shown in the past literature to selectively reduce carbon dioxide to hydrocarbons. However, due to the lack of systematic studies on time-scale resolved spectral features, the selectivity resulted from the surface atomic characteristics of the copper catalyst– could be copper metal or copper oxide, along the reaction path remains uncertain. This study introduces an X-ray absorption spectrum with a few seconds resolution scale to observe the chemical state evolution of catalysts in reactions. Under specific experimental conditions, it was observed that the average oxidation state consisted of the mixed oxidation states on the surface of copper material could be stably maintained on the electrode surface, thus steadily increasing the effective yield of ethanol. Combined with the first-principles simulations, the catalytic reaction pathway of CO2 conversion can be derived step-by-step and verified by experimental observations.