Self-consistent periodic density function theory is employed to investigate formaldehyde (CH2O) oxidation by platinum supported on perfect anatase TiO2(101) surface in the presence of adsorbed oxygen or hydroxyl species. The adsorption structures and energies of all possible intermediates involved are investigated to map out the reaction network. Our results show that the primary intermediates, i.e., CH2O, CH2O2, CHO2, CHO, and CO2, prefer to adsorb at the Pt–5cTi bridge site, which is found as the most active site on Pt/TiO2, and formaldehyde directly dehydrogenates through the pathway of CH2O → CHO → CO, while the reaction pathway of CH2O → CH2O2 → CHO2 → CO2 is favorable in the presence of oxygen. In the latter process, the decomposition of formate is the rate-limiting step due to its relatively high decomposition barrier. Energy barrier decomposition analysis is used to elaborate the promotion effects resulting from the coadsorbed oxygen and hydroxyl on the C–H bond scission of CH2O and CH2O2. The theoretical work sheds new light on the title reactions and can serve as a theoretical approach to the catalysis mechanisms of metal oxide supporting transition metals with small organic molecules.