Ag promoted Cu-SAPO-34 has been developed to improve the low-temperature hydrothermal stability of Cu-SAPO-34 for the NH3-SCR reaction. Kinetic tests show that the addition of Ag into Cu-SAPO-34 doesn't change the reaction mechanism. The ion-exchanged Ag species, which decrease the Bronsted acid density and provide more Lewis acid sites with moderate acidity, have a protective effect on the SAPO framework against low temperature steaming treatment (LTST). Higher surface Cu content is observed for CuAg2.25-SAPO-34, although CuAg2.23-SAPO-34 and Cu-SAPO-34 have similar content of isolated Cu ions. In situ DRIFTS studies demonstrate, for the first time, the NH3 molecules adsorbed on the weak/moderate acid sites (independent of the acid types such as Bronsted acid or Lewis acid) are more active than on the strong acid sites for the low-temperature SCR reaction. Compared with the original catalyst Cu-SAPO-34, the enhanced SCR activity of CuAg2.25-SAPO-34 after LTST is ascribed to the better preservation of total acid sites, higher moderate acid density (Lewis acid sites) and larger amount of surface Cu species on the CuAg2.25-SAPO-34 catalyst.