Abstract: In view of the poor resistance of manganese oxide octahedral molecular sieve (OMS-2) catalyst to sulfur in the oxidation of elemental mercury, CeO₂ was used to modify the OMS-2 catalyst. The mechanism for the enhancement of the resistance of the OMS-2 catalyst to sulfur by modifying with CeO₂ was investigated, with the help of thermodynamic analysis, fixed-bed reaction test and various characterization methods like nitrogen sorption, XRD ICP and XPS. The results indicate: The OMS-2 catalyst modified by Ce has a large surface area and void-rich structure, which can adsorb more Hg⁰ through chemical adsorption; More Mn defects are formed on the OMS-2 catalyst through modifying with Ce, leading to an increase in the electron mobility, the proportion of adsorbed oxygen (O_β) species, and the density of catalytically active sites; The Ce-modified OMS-2 catalyst can quickly re-oxidize the reduced Hg⁰ or HgSO₄ species (facilely formed on the pristine OMS-2 catalyst surface in the presence of SO₂) to HgO, which can then improve the apparent Hg⁰ oxidation efficiency. The results should be helpful for the development of high-performance anti-thio catalysts for mercury oxidation.