Single-atom catalysts have high catalytic activity due to their unique quantum size effects and optimal atom utilization. Herein, visible-light-responsive photocatalysts were designed by coupling CdS with graphene quantum dots (GQDs) and platinum single atoms (PtSAs). GQDs and PtSAs were successively loaded on ultrathin CdS nanosheets through freeze-drying and in-situ photocatalytic reduction. The synergistic effect between PtSAs and GQDs results in superior photocatalytic activity with a hydrogen production rate of 13488 μmol h^-1 g^-1 as well as the maximum apparent quantum efficiency (AQE) of 35.5% in lactic acid aqueous solution, which is 62 times higher than that of pristine CdS (213 μmol g^-1 h^-1). The energy conversion efficiency is ca. 13.05%. As a photosensitizer and an electron reservoir, GQDs can not only extend the light response of CdS to the visible-light region (400-800 nm), but also promotes the separation of photoinduced electron-hole pairs. Meanwhile, PtSAs, with unique electronic and geometric features, can provide more efficient proton reduction sites. This finding provides an effective strategy to remarkably improve photocatalytic H₂ production performance.