High metal dispersion supported catalysts have attracted extensive attention due to their high atom utilization and excellent activities in various catalytic reactions. In this work, Pt/Y₂O₃-HT was synthesized via a wet-impregnation method modified by hydrothermal process. Compared with Pt/Y₂O₃ without hydrothermal modification, CO oxidation activity of Pt/Y₂O₃-HT was significantly improved. The comprehensive experiment results demonstrate that hydrothermal aging at 200 °C could enhance basicity of yttria support by increasing the number of strong basic sites (low coordination surface O²⁻) and greatly improve dispersion of platinum at atomic level (about 30% and 82% platinum single atom for Pt/Y₂O₃ and Pt/Y₂O₃-HT, respectively). Moreover, in-situ DRIFTS results further indicate that hydrothermal treatment could alleviate CO adsorption rate on Pt atoms and clusters to provide more site for the adsorption of oxygen. Our work may provide a new insight for the mechanism of the effect of hydrothermal pretreatment on CO oxidation.