Novel, environmentally-benign catalysts for selective catalytic reduction of NOx were prepared by citric method through introducing transition metal elements (Ce, Cu and Co) into iron oxide. The physical-chemical properties of different catalysts were investigated by the characterization technologies like N2-physisorption, XRD, NH3/NOTPD and H2-TPR. The results indicated that the introduction of transition metal elements increased the specific surface area and adsorption ability for reactants (NH3 and NOx). The redox capacity for the doped catalysts was improved at the same time. These characteristics all contributed to the improvement of catalytic performance. The CoFeOx catalyst exhibited the widest temperature window for SCR reaction, and the CeFeOx catalyst showed the most obvious decline of NOx conversion with the elevation of temperature above 250 °C. Water vapor inhibited the SCR activity at low temperatures and relieved the decline of NOx conversion at higher temperatures. Meanwhile, the formation of N2O was inhibited. The pretreatment of SO2 leaded to the sulfation of the active species for different catalysts. The decline of redox capacity and the reduction of active nitrate adsorbed species accounted for the serious loss of SCR activity at low temperatures. The abundant surface acid sites brought by the sulfation process might be the main reason for good SCR activity in the medium temperature range.