In this work, we systematically studied the mechanism for the enhanced photocatalytic activities of TiO2-graphene composites by using density functional theory (DFT) calculations. The studied composites include: TiO2-pristine graphene, TiO2-graphene with defect, as well as TiO2-graphene oxide. The results from geometry optimization can reveal information about the interface structure and anchoring orientation of the composites. The calculated electronic properties including total and difference charge density, as well as charge population, demonstrate the polarization and electron redistribution for the composites. Projected density of states and energy bands can provide some useful information about the photocatalytic mechanism involving the electrons excitation from the O-2p orbital on the valence band to the C-2p on the conduction band maximum for the composites. The results of our study can provide some useful information for understanding the detailed molecular mechanism of the better performance of composites compared to the individual components.