Photoelectrochemical (PEC) cells fabricated using environment-friendly colloidal quantum dots (QDs) are promising optoelectronic devices for future practical solar-to-hydrogen conversion. However, the majority of current eco-friendly QDs-based PEC cells exhibited low efficiency mainly due to the charge trapping at QDs’ surface states and interfacial recombination processes in devices. Here, eco-friendly AgInS2 (AIS) QDs-based PEC cells passivated with variable ZnS layers were fabricated and the effects of ZnS surface passivation on corresponding device performance were investigated. It is demonstrated that optimizing the thickness of the ZnS passivation layers can largely suppress the charge trapping/recombination and enhance the electron injection efficiency in the PEC devices, leading to a saturated photocurrent density of ∼5.7 mA/cm2 under standard AM 1.5 G solar illumination. Increasing the thickness of ZnS passivation layers can further inhibit the photocorrosion and give rise to higher device stability. These results indicate that ZnS surface passivation is a facile and efficient technique to boost the performance of eco-friendly QDs-based PEC cells.