Exploiting novel and high-performance electron-cocatalysts without noble metallic element is of great significance for photocatalytic H-2-evolution reaction. Molybdenum sulfide is one of the promising candidates of such electron-cocatalysts, but its present performance is intrinsically restrained by the scarce active sites of unsaturated S atoms. In this study, amorphous MoSx (a-MoSx) nanoparticles were directly anchored on the g-C3N4 surface by an adsorption-in situ transformation method with the aim of improving photocatalytic H-2-evolution activity. It was found that compared with the crystalline molybdenum sulfide (c-MoS2), the a-MoSx cocatalyst clearly exhibited more unsaturated active S atoms due to its highly irregular arrangement structure. Photocatalytic experimental results suggested that the H-2-evolution activity of g-C3N4 photocatalyst could be obviously improved by loading a-MoSx cocatalyst, which is obviously higher than that of unmodified g-C3N4 and c-MoS2/g-C3N4. More importantly, in addition to the g-C3N4, the amorphous MoSx could also work as the efficient electron cocatalyst to greatly enhance the photocatalytic performance of conventional H-2-evolution materials such as TiO2 (a typical UV-light photocatalyst) and CdS (a typical Vis-light photocatalyst). On the basis of the present results, an electron-cocatalyst mechanism of amorphous MoSx was proposed to account for the improved photocatalytic H-2-evolution activity, namely, the amorphous MoSx can provide more unsaturated active S atoms as the efficient active sites to rapidly capture protons from solution, and then promote the direct reduction of H+ to H-2 by photogenerated electrons. Considering its low cost and high efficiency, the amorphous MoSx cocatalyst would have great potential for the development of high-performance photocatalytic materials used in various fields. (C) 2016 Elsevier B.V. All rights reserved.