g-C3N4/ZnO heterojunction composites were in situ prepared through the one-pot approach. The photocatalytic activities of the samples were evaluated by the photodegradation of methylene blue (MB) under visible light irradiation and the mechanism of g-C3N4/ZnO photocatalytic reaction was investigated. XRD, FT-IR, SEM and UV-Vis DRS were used to characterize the obtained composites. The results show that the heterojunctions are formed by the tight coupling between g-C3N4 and ZnO, which improves the separation efficiency of electron-hole pairs and have stronger light response in visible region. With the optimal g-C3N4 mass fraction of 19%, the catalytic reaction kinetic constants of MB solution degradation was 0.0206min-1, which was 3.8 times that of pure g-C3N4. The regenerated catalyst still kept good catalystic performance after recycling 5 times.