The ultrathin TiO2 nanosheets (TiO2-U) with abundant defects were successfully fabricated as substrates to support well-dispersed Pt nanoparticles with low metal loading by photochemical route (P), with impregnated Pt/TiO2-U, Pt/TiO2-bulk and photoreduced Pt/TiO2-bulk as control samples, to focus on studying the inductive effect of oxygen vacancy-rich ultrathin TiO2 on highly dispersed Pt nanoparticles, and their synergetic promotional effect for CO2 photoreduction. As expected, P-Pt/TiO2-U exhibited excellent photocatalytic efficiency for the selective conversion of CO2 into CH4 and CO. The ultrathin TiO2 nanosheets with a large number of low coordinative sites and ultralarge surface area promoted the rate of electron-transfer. The ultrafine Pt nanoparticles induced by photochemical strategy facilitated the efficient separation of electron-hole pairs. Furthermore, the synergy of metal and support improved the adsorption ability of CO2. These three factors were considered to drive jointly the enhancement of catalytic performance in this system. This work offers deep insights for the design of highly efficient catalysts with coordinatively unsaturated sites for CO2 photoconversion in the presence of H2O.