Effective dechlorination, as well as high dispersion, is necessary in the Pt nanocatalyst preparation from cheap chlorine-containing precursors like H₂PtCl₆. We herein report a facile approach to direct preparation of Pt nanocatalysts from oxidative dechloridation of the TiO₂ (P25) supported H₂PtCl₆ by O₂ plasma. Oxygen plasma can completely dechlorinate, enhance the dispersion of Pt nanoparticles, and create large number of low-coordinated sites, thus leading to the highest activity in WGS reaction. At 200 ^oC, the O₂ plasma treated Pt nanocatalyst achieves CO reaction rate of 1.07 mmol·mg⁻¹·h⁻¹ in WGS reaction, which is significantly higher than that of H₂ and N₂ plasmas treated samples. The oxidative dechloridation mechanism of O₂ plasma and structure–activity relationship of the supported Pt nanocatalysts were demonstrated.