In this study, the experiments combined with activity tests and characterizations were conducted to investigate the influence of hydrothermal aging (HTA) on the reactivity of NH₃-SCR over a Cu-CHA catalyst. The results showed that the NO_x conversion efficiency can reach more than 80% below 225 ℃ in the reaction between NH₃ and NO+O₂ after hydrothermal aging, but it increases the N₂O production. The wave spectrum of in-situ DRIFTs confirmed the notable differences in the NH₃-SCR mechanism between Cu species of ZCu(OH) and Z₂Cu at the same reaction conditions. The NH₃-SCR reaction occurring on ZCu(OH) followed the ‘Langmuir-Hinshelwood’ (L-H) mechanism, where NH₃ combined with products that oxidize NO to NO₂ via dimerization and disproportionation reactions to produce the key intermediates as NH₄NO₂ and HONO. However, the entire NH₃-SCR on Z₂Cu followed the ‘Eley-Rideal’ (E-R) mechanism, in which adsorbed NH₃ effectively reacted with gas-phase NO to form H₂NNO and then produced N₂.