The MoS2/g-C3N4 heterojunction composite (marked as MoS2/g-C3N4-H) was successfully prepared via calcining the melamine coated MoS2 nano-sphere, where the thin g-C3N4 nano-sheets were tightly grown on the surface of MoS2 nano-sphere to form MoS2 inlaying in g-C3N4 heterostructure. The detailed charge transfer mechanism was discussed by combining theoretical calculation and experiments, in which the intrinsic cause of photo-generated charge separation and transfer was determined as the directional built-in electric field driven by different Fermi levels of MoS2 and g-C3N4. Comparatively, the enhanced photo-catalytic performance and stability of the sample were assessed by degrading the Rhodamine (RhB) and reducing the Dichromate (Cr6+) solutions under the irradiation of the simulated sunlight, which could be attributed to the widened spectral absorption range and improved electron-hole separation rate. Based on above results, the photo-catalytic mechanism involving redox reactions was also clearly proposed.