This research has developed a photocatalytic reactor that includes circulating water, light, and a temperature control system. CeO2/g-C3N4 composites with high photocatalytic activity and stability were synthesized by a simple and facile hydrothermal method. The obtained photocatalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). It was found that in the CeO2/g-C3N4 composites, the CeO2 nanoparticles were homogeneously cubic in shape (from 3 to 10 nm) and were evenly dispersed on the surface of the g-C3N4. At constant temperature (30 °C), 5% CeO2/g-C3N4 photocatalyst showed the best photocatalytic activity for degrading organic dye methylene blue (MB) under visible light irradiation. The photocatalytic reaction for degrading MB followed first-order kinetics and 5% CeO2/g-C3N4 exhibited a higher apparent rate of 1.2686 min(-1), 7.8 times higher than that of the pure g-C3N4 (0.1621 min(-1)). In addition, it was found that 5% CeO2/g-C3N4 had a new property that it could be used as a sensor for the determination of trace amounts of Cu(2+). Such unique design and one-step synthesis, with an exposed high-activity surface, are important for both technical applications and theoretical investigations.