Two-dimensional graphitic carbon nitride (g-C3N4) has been widely explored as a promising photocatalyst for solar CO2 conversion. However, rapid charge recombination and low visible-light utilization are severely detrimental to photocatalytic CO2 conversion. Zero-dimensional/two-dimensional (OD/2D) heterostructures are considered the promising materials with size tunability and enhanced charge separation efficiency for photo catalysis. Herein, a OD/2D heterostructure of oxygen vacancy-rich TiO2 quantum dots confined in g-C3N4 nanosheets (TiO2-x/g-C3N4) was prepared by in-situ pyrolysis of NH2-MIL-125 (Ti) and melamine. Charge dynamics analysis by time-resolved photoluminescence (tr-PL) and femtosecond and nanosecond pump-probed transient absorption (TA) spectra revealed that charges transfer occured from 2D-g-C3N4 to OD-TiO2 at an ultrafast sub picosecond time scale ( < 1 ps) through the intimate interface. The overall fast decay of the charge carriers was attributed to interfacial charge transfer, which was accompanied by recombination relaxation mediated by shallow trapped sites. Ultrafast interfacial charge transfer greatly promoted charge separation and electrons in shallow trapped sites were easily trapped by CO2. In addition, combining with the synergetic advantage of strong visible light absorption, high CO2 adsorption and large surface area, TiO2. /g-C3N4 exhibited a superior CO evolution rate of 77.8 pmol g(-1) h(-1), roughly 5 times that of pristine g-C3N4 (15.1 pmol g(-1) h(-1)). This work provides in-depth insights into optimizing the heterojunction for robust solar CO2 conversion.