In this work, plasmonic Au/SnO2/g-C3N4 (Au/SO/CN) nanocomposites have been successfully synthesized and applied in the H2 evolution as photocatalysts, which exhibit superior photocatalytic activities and favorable stability without any cocatalyst under visible-light irradiation. The amount-optimized 2Au/6SO/CN nanocomposite capable of producing approximately 770 μmol g−1 h−1 H2 gas under λ > 400 nm light illumination far surpasses the H2 gas output of SO/CN (130 μmol g−1), Au/CN (112 μmol g−1 h−1), and CN (11 μmol g−1 h−1) as a contrast. In addition, the photocatalytic activity of 2Au/6SO/CN maintains unchanged for 5 runs in 5 h. The enhanced photoactivity for H2 evolution is attributed to the prominently promoted photogenerated charge separation via the excited electron transfer from plasmonic Au (≈520 nm) and CN (470 nm > λ > 400 nm) to SO, as indicated by the surface photovoltage spectra, photoelectrochemical I–V curves, electrochemical impedance spectra, examination of formed hydroxyl radicals, and photocurrent action spectra. Moreover, the Kelvin probe test indicates that the newly aligned conduction band of SO in the fabricated 2Au/6SO/CN is indispensable to assist developing a proper energy platform for the photocatalytic H2 evolution. This work distinctly provides a feasible strategy to synthesize highly efficient plasmonic-assisted CN-based photocatalysts utilized for solar fuel production.