CeO₂/TiO₂ catalyst is a promising material for realizing the integration of denitrification and mercury removal to reduce mercury emissions. Oxidation mechanism of Hg⁰ on CeO₂/TiO₂ (001) surface in the presence of HCl and O₂ was studied by density functional theory (DFT). The results indicated that Hg⁰ was physically adsorbed on CeO₂/TiO₂ (001) surface. As an important intermediate, HgCl was adsorbed on the surface of CeO₂/TiO₂ (001) utilizing enhanced chemisorption, while the adsorption energy of HgCl₂ was only –57.05 kJ/mol. In the absence of HCl, mercury oxidation followed Mars-Maessen mechanism with a relatively high energy barrier, and the product (HgO) was difficult to desorb, which hindered the reaction process. When HCl existed, reactive chlorine (Cl*) would be produced by the dissociation of HCl, and the mercury oxidation would follow Langmuir-Hinshelwood mechanism. The co-existence of HCl and O₂ had no significant effect on the adsorption of Hg⁰, but reduced the reaction energy barrier and the final product (HgCl₂) more easily desorbed from the catalyst surface. In addition, two complete cyclic reaction pathways for catalytic oxidation of Hg⁰ on CeO₂/TiO₂ (001) surface were constructed to clarify the detailed reaction process.