Au/g-C3N4 composite is prepared through protonation of g-C3N4, followed by electrostatic adsorption of AuCl4- and subsequent chemical reduction. The homogeneous deposition of Au nanoparticles with an average diameter of 4.1 nm on g-C3N4 is confirmed by TEM measurement. Compared with pure g-C3N4, Au/g-C3N4 composite exhibits a significant improvement in the photocatalytic activity for H-2 production from water in the presence of triethanolamine as sacrificial electron donor under visible light irradiation (lambda > 420 nm). Significantly, the rate of H-2 production increases more than 22 times by adding fluorescein in the photocatalytic reaction system. It is more than 25 times higher than that of protonated g-C3N4 in the presence of fluorescein. Based on our experimental results, the mechanism of photocatalytic activity for H-2 evolution over fluorescein-sensitized Au/g-C3N4 nanocomposite is accordingly proposed. The highly-improved performance is ascribed to the efficient transfer of photo-generated electrons among photo-excited fluorescein molecule, g-C3N4 and Au, which is supported by photoluminescence spectra and transient photocurrent responses.