The kernel of photocatalytic technology is to exploit photocatalytic materials with high efficiency, stability and easy recovery. Herein, we designedly prepare a magnetic quantum dot (Fe3O4 QDs) as co-catalyst to modify Z-scheme Bi2O4/g-C3N4 (Bi/CN) composite photocatalyst via combining calcination with hydrothermal method. The morphology, chemical composition and physicochemical properties of photocatalysts are systematically analyzed by a series of characterization means. The experimental results explicitly render that the optimum Fe3O4-QDs/Bi2O4/g-C3N4 (FeQDs/Bi/CN) has outstanding photocatalytic performance than that of Bi2O4, g-C3N4 and Bi2O4/g-C3N4 (Bi/CN), which is chiefly imputed to the synergistic effect of Z-scheme heterojunction system and Fenton reaction. The Z-scheme heterojunction is conducive to accelerate charge carrier separation and makes electrons and holes retain higher redox ability. Fe3O4 QDs can promote visible light absorption and generate more hydroxyl active (center dot OH) substances through Fenton reaction. Furthermore, the magnetic photocatalyst is liable to separate under the action of external magnetic field, which ensures that is convenient for industrial application. The mechanism of photocatalytic degradation is discussed in detail by capturing experiments, ESR and liquid fluorescence detection. This work unveils more possibilities for modifying Z-scheme heterojunction system to improve photocatalytic performance.