In the past decade, density functional theory (DFT) calculations have been employed to study the mechanism of electrochemical CO2 reduction reactions. However, the lack of understanding of the CO2 chemisorption states, proton-coupled-electron-transfer (PCET) steps, and dynamic redox reactions of the electrode surface has limited the reliability of these simulations. The *OCHO and *COOH species are widely recognized as the key intermediates for the formic acid and carbon monoxide production, respectively. However, the comparison between the binding energies of *OCHO and *COOH cannot directly indicate the reaction trends. In this work, we propose that the energy difference between *COOH on the neutral and extra-electron substrates, in the form of [ΔG(*COOHe) – ΔG(*COOH)], can serve as a descriptor for the electrochemical CO2 reduction selectivity. In addition, the computational hydrogen electrode (CHE) model is revised by applying the previously studied charged species. The noninteger charge-transfer (NICT) model is used for the calculation of energy profile at a certain potential, which can have a good prediction of the potential-limiting step. The surface oxide of metal electrodes is found to play a key role in modulating the selectivity and improving the electron transfer to CO2.