Improving SO2/H2O resistance at low temperatures is a key challenge for industrial applications of NH3-SCR catalysts. Herein, a novel catalyst with remarkable low-temperature NH3-SCR activity and strong SO2/H2O resistance was designed by loading MoOx protective layer over porous kit-CeO2. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) coupled with basic characterization techniques were utilized to further elucidate the underlying mechanisms, and a dynamic balance between the deposition and decomposition of ammonia bisulfate was captured over porous kit-CeO2. Highly dispersed MoOx species pre-adsorbed with NH3 significantly curtailed the adsorption of NOx molecules, and the catalytic reaction over MoOx/kit-CeO2 primarily followed by the Eley-Rideal mechanism. At 250 °C, in the presence of SO2 and H2O, the Ce-O-Mo pairing site was almost unaffected and the super SO2- and H2O-tolerant property originated from the facilitated decomposition of ammonium bisulfate (ABS) and the shielding effect of Mo protective layer on SO2/H2O. This study provided an important guidance to the application of ceria-based catalysts for environmental control within low-temperature operations in non-power industries such as glass, steel, cement manufacturing, etc. Display omitted •A catalyst with good NH3-SCR activity was designed by loading MoOx over kit-CeO2.•MoOx/kit-CeO2 shows strong SO2/H2O resistance.•The NH3-SCR reaction primarily followed by the Eley-Rideal mechanism.•MoOx/kit-CeO2 can facilitate decomposition of ammonium bisulfate.•MoOx protective layer on SO2/H2O shows the shielding effect.