In this research, Pt-loaded TiO2 was prepared by photodeposition used for acetic acid photocatalytic reforming. CO2 was injected into an acetic acid solution for removing O2 and inducing the formation of CO32–. The production rate of CH4 were higher in a CO2 atmosphere than that in an Ar atmosphere (about 69%). According to electron spin resonance (ESR) analysis, more •CH3 radical was produced in CO2 atmosphere, suggesting that CO2 introduced in acetic acid solution can better promote acetic acid reforming to CH4. On the contrary, the production rate of H2 is higher under an Ar atmosphere than that under CO2, suggesting a peroxidation of acetic acid transferred to H2.