Phosphorus were employed to investigate the influences on the acid sites and reaction mechanism of FeTiOx SCR catalyst. The introduction of phosphorus inhibited the SCR activity by suppressing the redox property, decreasing the surface acidity and reducing the surface area. In particular, phosphorus preferentially bonded to the electron-deficient Ti4+ and Fe3+ cations on the surface as P-O-Fe and P-O-Ti, thereby blocking the Lewis acid sites (Fe-NH3 or Ti-NH3). Although phosphate provided additional Bronsted acid sites (P-O-NH4+), the larger decrease of Lewis acid sites results in the decrease of total surface acidity. The newly formed Bronsted acid sites showed less SCR active at low temperature and exhibited high thermal stability and reactivity at high temperature, which leads to a new pathway following the L-II mechanism and an enhancement of high temperature performance.