A Cu-SSZ-13 molecular sieve catalyst was synthesized by the ion exchange method, which had a high NO_x conversion rate and N₂ selectivity. The NO_x conversion rate, N₂ selectivity and NH₃ adsorption performance were between the fresh and aged catalysts and the effect of hydrothermal aging on NH₃-SCR performance of Cu-SSZ-13 catalyst was analyzed after the preparation of aged catalysts through 16 h hydrothermal aging treatment at 700 ℃ and 800 ℃ respectively. The physicochemical properties of catalyst were characterized by using XRD, BET and SEM methods. The NH₃ and NO adsorption and reaction mechanism of catalyst before and after aging were investigated by the in situ DRIFTS. The results show that the hydrothermal aging will lead to the deterioration of Cu-SSZ-13 catalyst crystallinity and the reduction of specific surface and pore volume. The catalytic activity of Cu-SSZ-13 zeolite catalyst is still good and the skeleton structure is wellsustained after hydrothermal aging treatment at 700 ℃, but it becomes poor at 800 ℃, the catalyst skeleton collapses and more Cu_xO species forms on the active surface. The high-temperature hydrothermal aging leads to the reduction of NH₃ absorbed in Br-nsted acid location and the increase of NH₃ absorption on the catalyst surface. The total NH₃ adsorption reduces with the increase of water thermal aging temperature. In addition, the high-temperature hydrothermal aging leads to the weakening of NO adsorption capacity and most of the NO adsorbed on the surface of catalyst are oxidized to nitrate. The proportions of various nitrates are almost the same on the surface of fresh and aged catalyst. Accordingly, the hydrothermal aging does not change the catalytic mechanism of Cu-SSZ-13 molecular sieve catalyst, which make it possible to further improve the hydrothermal aging performance of catalyst.