This study aims to investigate the enhanced low temperature denitration activity and promotional mechanism of a cerium-based catalyst through copper modification. In this paper, copper and cerium oxides were supported on TiO₂–ZrO₂ by an impregnation method, their catalytic activity tests of selective catalytic reduction (SCR) of NO with NH₃ were carried out and their physicochemical properties were characterized. The CuCe/TiO₂–ZrO₂ catalyst shows obviously enhanced NH₃-SCR activity at low temperature (<300 °C), which is associated with the well dispersed active ingredients and the synergistic effect between copper and cerium species (Cu²⁺ + Ce³⁺ ↔ Cu⁺ + Ce⁴⁺), and the increased ratios of surface chemisorbed oxygen and Cu⁺/Cu²⁺ lead to the enhanced low-temperature SCR activity. The denitration reaction mechanism over the CuCe/TiO₂–ZrO₂ catalyst was investigated by in situ DRIFTS and DFT studies. Results illustrate that the NH₃ is inclined to adsorb on the Cu acidic sites (Lewis acid sites), and the NH₂ and NH₂NO species are the key intermediates in the low-temperature NH₃-SCR process, which can explain the promotional effect of Cu modification on denitration activity of Ce/TiO₂–ZrO₂ at the molecular level. Finally, we have reasonably concluded a NH₃-SCR catalytic cycle involving the Eley–Rideal mechanism and Langmuir–Hinshelwood mechanism, and the former mechanism dominates in the NH₃-SCR reaction.