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Ce-based selective catalytic reductions with an NH₃ (NH₃-SCR) catalyst have emerged as a focal point in denitrification catalyst research. However, the correlation between the structural characteristics of Ce-based catalysts and the influence of CeO₂ nanoparticle size on SO₂ resistance remains unclear. CeO₂ nanospheres with different sizes of less than 10 nm were synthesized, and a series of supported CeO₂/SBA-15 catalysts were prepared according to the 10 nm pore size of SBA-15. These catalysts were used to explore the influence of the size of the CeO₂ nanospheres on these catalysts, specifically on their SO₂ resistance in NH₃-SCR reactions. With the increase in size, their SO₂ resistance became stronger. The results of NH₃-TPD, H₂-TPR, and XPS indicated that the catalyst with the largest particle size had the lowest adsorption of SO₂, which was attributed to more acid sites and a mutual effect between Si and Ce, resulting in the best SO₂ resistance. It was also observed that there was less sulfate deposition on the catalyst by thermogravimetric analysis. In situ DRIFTs revealed that after SO₂ poisoning, the NH₃-SCR reaction on the catalyst predominantly follows the E-R mechanism. This study offers recommendations for the development of Ce-based SO₂-resistant NH₃-SCR catalysts, specifically focusing on the synthesis and interaction of nanomaterials.