The construction of efficient semiconductor/cocatalyst heterojunction is a promising strategy to promote the photocatalytic H2-production efficiency from water reduction. Herein, a novel Cu2(OH)2CO3/Zn0.5Cd0.5S heterojunction composite with superior photocatalytic H2-generation activity and stability was prepared via a facile in situ synthetic route, in which Cu2(OH)2CO3 served as cocatalyst stimulating the photocatalytic H2-evolution performance of Zn0.5Cd0.5S. The Cu2(OH)2CO3/Zn0.5Cd0.5S heterojunction photocatalyst containing 5 wt% of Cu2(OH)2CO3 exhibited a prominent H2-evolution efficiency of 275.7 μmol h−1, which is superior to the noble metal Pt-modified Zn0.5Cd0.5S photocatalyst (237.3 μmol h−1) and much higher than bare Zn0.5Cd0.5S (90.8 μmol h−1). The active Cu+/Cu0 species generated during the photoreaction process is responsible for the prominent H2-production activity of the heterojunction photocatalyst, which plays triple roles in enhancing the photoactivity of Zn0.5Cd0.5S via accelerating the charge separation, decreasing the overpotential of H2-generation and improving the reduction ability of photoinduced electrons. Moreover, the formed Cu+/Cu0 species during photoreaction can be readily oxidized back to Cu2(OH)2CO3 upon exposure to air, thus restores the photoactivity and therefore enables good reusability of the Cu2(OH)2CO3/Zn0.5Cd0.5S heterojunction photocatalysts. This work provides a new insight into the fabrication of Cu2(OH)2CO3-assisted heterojunction photocatalysts with the highly stable and efficient performances for solar-to-chemical energy conversion. Display omitted •A novel binary Cu2(OH)2CO3/Zn0.5Cd0.5S composite was successfully synthesized.•The trifunctional roles of Cu2(OH)2CO3 cocatalyst were discussed.•Mechanism of the enhanced photocatalytic activity was illuminated.