The impact of SO2 poisoning on Cu-SAPO-18 physicochemical properties and its NH3-SCR reaction mechanism was investigated. After exposing Cu-SAPO-18 to SO2+O-2 for 40 min, the sulfite and sulfate species were detected on Cu-SAPO-18 surface, which caused a small decrease in BET surface area and micropore volume, the increase in NH3 storage and the deactivation of some isolated Cu2+ sites. These could be interpreted by the slight micropore blocking, interaction between sulfates and NH3 forming new NH4+ species and the formation of Cu-sulfate, respectively. Additionally, it was observed that isolated Cu2+ ions in pear-shaped cavity of AEI structure, which are beneficial to good low-temperature deNO(x) activity, were more severely poisoned by SO2 than Cu2+ in the double 6-membered ring (D6R). Accordingly, the NH3-SCR activity below 350 degrees C was restrained by SO2. Moreover, the in-situ DRIFTS results indicate that Cu-SAPO-18 catalyzed NH3-SCR reaction predominantly proceeds through Langmuir-Hinshelwood (L-H) mechanism at low temperature. SO2 poisoning inhibited the adsorption of NO + O-2 and NH3 on Cu2+ sites, thereby decreasing the formation of Cu-NOx and Cu-NH3 intermediates, evidently, the L-H mechanism was suppressed by SO2. Interestingly, the deactivated Cu-SAPO-18 could be regenerated under SCR gas at 550 degrees C and resumed at least most NH3-SCR activity.