As a result of WNTE regulations and the introduction of close-coupled aftertreatment systems, exhaust purification at high temperatures in commercial vehicles has become increasingly important in recent years. In this report, we improve the prediction accuracy for NOx conversion at high temperatures in the kinetic model of conventional Cu-selective catalytic reduction (Cu-SCR). Reaction rate analysis indicated that the rate of NH3 oxidation was extremely low compared to the rate of standard SCR. We found that NOx concentration-dependent NH3 oxidations (termed NOx-assisted NH3 oxidations) were key to the rate of NH3 oxidation. The output of the improved Cu-SCR kinetic model was in agreed with experimental results obtained from the synthetic gas bench and engine dynamometer bench. We analyzed the contribution of each reaction to NH3 consumption during Cu-SCR. Under NH3 + NO + O2, standard SCR was dominant at low temperature. At high temperatures, the rate of NO-assisted NH3 oxidation increased, and this reaction competed with standard SCR. Under NH3 + NO + NO2 + O2, fast SCR, NH4NO3 formation, and standard SCR were the dominant reactions at low temperature. With increasing temperature, NO2-assisted NH3 oxidation competed with the other reactions, resulting in a decreased NOx conversion.