Here,we explore the enhanced photocatalytic mechanism for the hybrid g-C3N4/MoS2 nanocomposites for the first time by performing extensive density functional theory calculations.The calculated band alignment between g-C3N4 monolayer and MoS2 sheet clearly reveals that the conduction band minimum and valence band maximum of g-C3N4 monolayer is higher about 0.83 eV and 0.15eV than that of MoS2 sheet,respectively.This predicted type-Ⅱ band alignment ensures the photogenerated electrons easily migrating from g-C3N4 monolayer to MoS2 sheet,and leads to the high hydrogen-evolution reaction activity.The charge transfer between MoS2 and g-C3N4 results in a polarized field within the interface region,which will benefit the separation of photogenerated carriers.The calculated optical absorption curves verify that this proposed layered nanocomposite is a good light-harvesting semiconductor.Moreover,a g-C3N4 bilayer covered on MoS2 sheet also displays desirable properties.These findings indicate that MoS2 sheet is a promising candidate as a non-noble metal co-catalyst for g-C3N4 photocatalyst,and also provide useful information for understanding the observed enhanced photocatalytic mechanism in experiments.