SCR activity at low temperature over iron oxide catalyst was prominently optimized by adding manganese and magnesium. Fe0.7Mn0.15Mg0.15Oz(n(Mn)/[n(Fe)+n(Mn)+n(Mg)])=0.15 and n(Mg)/[n(Fe)+n(Mn)+n(Mg)]=0.15) presented better performance in the low temperature SCR and NOx conversion of 90% could be achieved over 125 °C. Meanwhile, part of manganese and magnesium oxides were highly dispersed on the catalyst surface in an amorphous phase to react with iron oxide to form solid solution. Manganese and magnesium dopants could optimize the pore structure and distribution of γ-Fe2O3 to enhance the surface area and pore volume. Moreover, O2 participated in SCR reaction at a faster rate than NH3. In addition, the effect of SO2 was proved to be irreversible, whereas the inhibition of H2O could be rapidly removed after its removal.