As a typical catalytic reaction model, CO catalytic oxidation has many practical applications in the gas purification. TiO2 supported Pt sub-nanoclusters have been prepared by introduced variable valence Co ion into one step flame spray pyrolysis process. Co2+ was oxidized to Co3+ in the high-temperature flame, the released electrons could transfer to the surface of Pt and suppressed the aggregation of Pt nanoclusters supported on TiO2, the average size of Pt nanoclusters reduced from 2.47 nm to 0.72 nm with only 1% Co2+ ion doping. Moreover, due to the presence of Co, surface oxygen species are also affected, these changes also lead to a significant increase in catalytic activity of CO oxidation. The temperature at 100% conversion has decreased from 120 °C to 70 °C, the TOFs increased by an order of magnitude. In addition, in situ DRIFTS was also used to investigate the cause of a significant increase in activity, and it shows that adsorbed CO species on Pt can be desorbed more easily because of the electron transfer between Pt and Co species.