Ternary CdSe quantum dots (QDs)/graphene/TiO2 nanosheet composite photocatalysts were synthesized through precipitation by using hydrothermally prepared layered TiO2 nanosheets/graphene sheets as supporting matrix and CdSe QDs solution as precursor. During synthesis, anatase TiO2 nanosheets with exposed (001) facets were tightly grown on the surface of graphene sheets with face-to-face orientation, and CdSe QDs with an average size of 3.0 nm were uniformly and closely attached to the layered TiO2/graphene sheets through a molecular linker mercaptopropionic acid. The genuine structure and constituents of the prepared ternary CdSe QDs/graphene/TiO2 composites provide their great adsorptive capacity for aromatic dye on graphene, extended visible-light absorption range by CdSe QDs, and efficient charge separation by using TiO2 nanosheets as an electron mediator. Hence, CdSe QDs/graphene/TiO2 composites show high visible light photocatalytic activity and stability for the degradation of methylene blue in aqueous solutions. Moreover, ternary CdSe QDs/graphene/TiO2 composites show high photocatalytic antibacterial activity against Escherichia coil (E. coil) under visible light irradiation by generating active species, such as holes, center dot O2(-), and center dot OH. The proposed photocatalytic mechanism and some reactive active species were further confirmed by experiments on transient photocurrent responses, through the addition of the scavenger, and through the detection of center dot OH radicals.