Uniform mesoporous iron titanium (CT-FeTi) catalysts were synthesized by a CTAB-assisted process and exhibited good catalytic activity and H2O resistance when tested in the selective catalytic reduction (SCR) of NO with NH3 at low temperature. BET, TPD, TPSR and in situ DRIFTS were carried out to reveal the inhibition mechanism of H2O on SCR reaction and determine the role of CTAB in the improved H2O resistance during low-temperature SCR processes. Results showed that H2O inhibited the adsorption of NO on the surface of CT-FeTi catalysts, as well as the formation of the intermediate species (-NH2), which was produced by the reaction Fe3+ + NH3 -> Fe2+-NH2 + H+ CTAB acted as a "structural" and "chemical" promoter, not only optimizing the pore size to avoid being excessively enlarged in the presence of H2O, but also enhancing the adsorption of bridging nitrate and NH3 species on Lewis acid sites, thus improving the catalytic activity and H2O resistance.