Visible-light-driven photocatalytic hydrogen production has been an ongoing hot topic for clean and renewable energy, for which I–III–VI multinary metal sulfides play an important role owning to their unique ability of bandgap manipulation by composition. In this work, we present the controlled synthesis and hydrogen evolution property of a series of Ag:Zn-In-S quantum dots (QDs) in an effort to a more comprehensive understanding of Ag doping. With increasing ratio of Ag, the UV–vis absorption and photoluminescence wavelength was observed to be tuned in a wide range from light green to dark red. With increased Ag doping, dramatic increase (over 70 times) of the photocatalytic activity was observed (maximized with Ag:In:Zn ratio of 1.5:10:5). More importantly, time-resolved photoluminescence study reveals that this synergistic enhancement effect of the photocatalytic activity by controllable Ag doping was correlated with the simultaneous bandgap narrowing and carrier lifetime elongation, which contribute to the enhanced visible light absorption and charge separation, repectively. The enhanced charge separation upon Ag doping were further proved by photocurrent and electrochemical impedance spectra measurements. A mechanism is proposed for this synergistic effect of photocatalysis by suitable Ag doping, where the long-lived deep donor-acceptor pair states play an important role. Our results provide an interesting view and useful guideline for photocatalyst design using the narrow bandgap multinary sulfides.