CO2 hydrogenation over catalysts is a potentially exciting method to produce fuels while closing the CO2 cycle and mitigating global warming. The mechanism of this process has been controversial due to the difficulty in clearly identifying the species present and distinguishing which are reaction intermediates and which are byproducts. We in situ manipulated the independent formation and hydrogenation of each adsorption species produced in CO2 hydrogenation reaction over Ru/Al2O3 using operando diffuse reflectance infrared Fourier transformation spectroscopy (DRIFTS) and executed a novel iterative Gaussian fitting procedure. The adsorption species and their role in the CO2 hydrogenation reaction have been clearly identified. The adsorbed carbon monoxide (CO*) of four reactive structures was the key intermediate of methane (CH4) production. Bicarbonate (HCO3–*), formed on the metal–support interface, appeared to be not only the primary product of CO2 chemisorption but also a reservoir of CO* and consisted of the dominate reaction steps of CO2 methanation from the interface to the metal surface. Bidentate formate (Bi-HCOO–*) formed on Ru under a certain condition, consecutively converting to CO* to merge into the subsequent methanation process. Nonreactive byproducts of the reaction were also identified. The evolution of the surface species revealed the essential steps of the CO2 activation and hydrogenation reactions which were inevitably initiated from HCO3–* to CO* and finally from CO* to CH4.