Bayan Obo rare earth tailings contain Fe, Ce and other active elements conducive to catalytic denitration. As a natural mineral, it is environment-friendly and low-cost. It is a natural raw material for denitration catalyst, but the temperature window of rare earth tailings catalyst is narrow (350-450℃). In order to broaden the temperature window of rare earth tailings catalysts, a series of Ce-M (Nb, Co) modified rare earth tailings catalysts were prepared by hydrothermal method. Exploring the effect of element ratio and element type on denitrification performance. In particular, the experimental materials are Bayan Obo rare earth tailings, cerium nitrate, niobium oxalate and cobalt nitrate. The mixed solution of Bayan Obo rare earth tailings and cerium nitrate is made, poured into the reactor, and then placed in the muffle furnace, hydrothermal at 120℃ for 12h. After washing with water and drying at 80 ℃, the Ce modified rare earth tailings catalyst was prepared. The preparation methods of other modified rare earth tailings catalysts are the same. The modified rare earth tailings were characterized and analyzed by BET, XRD, XPS, H2-TPR and NH3-TPD. Finally, the NH3-SCR mechanism of Ce-Co modified rare earth tailings was explored by in situ DRIFTS technology. The steady-state reaction is carried out at 350℃ and the transient reaction is carried out at 50-400℃, which can analyze the existing forms of adsorbed NH3 and NOx species in the catalytic process and their changes with temperature. Therefore, the NH3-SCR reaction mechanism of Ce co modified rare earth tailings catalyst is analyzed. The results show that the maximum denitration efficiency of Ce-Nb (2:1) modified rare earth tailings is 85% at 300-400℃, and the denitration efficiency of Ce-Co (2:1) modified rare earth tailings can reach 90% at 250-400℃. The addition of Nb and Co improved the dispersion of CeCO3F on the catalyst surface and exposed more active adsorption sites. At the same time, the interaction between elements promotes the transfer of electrons. The existence of Nb5+ hinders the reduction of Ce4+, regulates the redox performance, and makes the Ce-Nb modified rare earth tailings have excellent N2 selectivity. The addition of Co improves the redox capacity of the catalyst and increases Co3+. Further, the adsorption strength of Br?nsted acid site is improved. NH4+ and Co3+-NH2 can react with NO first, form intermediate products such as NH3HNO and NH2NO. The surface of the catalyst follows both E-R mechanism and L-H mechanism, and E-R mechanism is dominant.