Bi4Ti3O12 square plates were synthesized via a hydrothermal route, and their growth process was systematically investigated. Carbon quantum dots (CQDs) were prepared using glucose as the carbon source, which were then assembled on the surface of Bi4Ti3O12 square plates via a hydrothermal route with the aim of enhancing the photocatalytic performance. XRD (X-ray powder diffraction), SEM (scanning electron microscopy), TEM (transmission electron microscopy), UV-vis DRS (diffuse reflectance spectroscopy), XPS (X-ray photoelectron spectroscopy), FTIR (Fourier transform infrared spectroscopy), PL (photoluminescence) spectroscopy, EIS (electrochemical impedance spectroscopy) and photocurrent spectroscopy were used to systematically characterize the as-prepared samples. It is demonstrated that the decoration of CQDs on Bi4Ti3O12 plates leads to an increased visible light absorption, slightly increased bandgap, increased photocurrent density, decreased charge-transfer resistance, and decreased PL intensity. Simulated sunlight and visible light were separately used as a light source to evaluate the photocatalytic activity of the samples toward the degradation of RhB in aqueous solution. Under both simulated sunlight and visible light irradiation, CQDs@Bi4Ti3O12 composites with an appropriate amount of CQDs exhibit obviously enhanced photocatalytic performance. However, the decoration of excessive CQDs gives rise to a decrease in the photocatalytic activity. The enhanced photocatalytic activity of CQDs-modified Bi4Ti3O12 can be attributed to the following reasons: (1) The electron transfer between Bi4Ti3O12 and CQDs promotes an efficient separation of photogenerated electron/hole pairs in Bi4Ti3O12; (2) the up-conversion photoluminescence emitted from CQDs could induce the generation of additional electron/hole pairs in Bi4Ti3O12; and (3) the photoexcited electrons in CQDs could participate in the photocatalytic reactions.