Understanding the role of different solvent molecules for practical solid-liquid heterogeneous photocatalytic reactions is critical for determining the pathway of the reaction. In this study, the operando nuclear magnetic resonance (NMR) method, combined with density functional theory (DFT) calculations, was employed to evaluate the control effect of solvent water in the photocatalytic reforming mechanism of methanol with a Pt-TiO2 catalyst. Results indicate that the presence of water effectively promotes the formation of the HCHO intermediate but inhibits the H2 evolution originating from the switch of the hydrogen source of the H2 formation from CH3OH to H2O. More interestingly, as detected directly in the ab initio molecular dynamics simulation, a small amount of H2O can dissociate, and the evolved -OH species at Ti5c site can greatly reduce the C-H activation barrier of -CH3O, contributing to the formation of oxidation products (e.g., HOCH2OH and CH3OCH2OH) on the Pt-TiO2 surface.