CeO2/TiO2 has been regarded as the promising catalyst used for the selective catalytic reduction (SCR) of NOx with NH3. The complete NH3-SCR reaction mechanism of NO over CeO2/TiO2 catalyst was systemically investigated using density functional theory (DFT) calculations. Based on the DFT-derived energetics, a skeletal reaction scheme including 20 elementary reaction steps was proposed for NH3-SCR reaction of NO over CeO2/TiO2 catalyst. The results indicate that NH3, NO, 02, and NO2 adsorption on CeO2/TiO2(110) surface is dominated by the chemisorption mechanism. The successive dehydrogenation process of NH3 is cleaved by NH2 dehydrogenation step. NH2 derived from NH3 dehydrogenation step can stably exist on CeO2/TiO2 catalyst surface and serve as a key intermediate in NH3-SCR reaction. The activation energy barrier of N2O formation is higher than that of N-2 formation, which is responsible for the excellent N-2 selectivity of CeO2/TiO2 catalyst. NO2 formation under the typical low -temperature SCR conditions can make a contribution to NO reduction over CeO2/TiO2 catalyst. Reaction pathway analysis illustrates that the dominant reaction pathway of NO reduction by NH3 over CeO2/TiO2 catalyst is a three-step process: (1) NH2 formation from NH3 dehydrogenation, (2) N-N coupling reaction, and (3) NH2NO decomposition. During NO reduction, NH2 formation from NH3 dehydrogenation is the rate-determining step of N-2 formation.