Different loading rates of composite photocatalysts Fe3O4/g-C3N4 were prepared by in situ precipitation method and were characterized by IR and XRD spectroscopies. Their photocatalytic performances were evaluated by the degradation of RhB under visible light irradiation. Fe3O4/g-C3N4 composites assisted by H2O2 shows higher photocatalytic performance than that of Fe3O4 or g-C3N4. As the dosage rate of Fe3O4 increases, the degradation rate is slightly increased. The Fe3O4/g-C3N4(0.1:1) composite has the best photocatalytic performance. It is inferred that the introduction of Fe3O4 is beneficial for the separation of photogenerated electrons and holes on the surface of the catalyst.