•A new ternary Zn0.5Cd0.5S/g-C3N4/RGO composite was successfully synthesized.•The synergetic effects of g-C3N4 and RGO on the photoactivities were discussed.•Mechanism of the enhanced photocatalytic activity was illuminated. In this work, a novel ternary composite photocatalytic material was constructed by in-situ deposition of 0D Zn0.5Cd0.5S nanoparticles onto the 2D g-C3N4 and reduced graphene oxide (RGO) surfaces via a simple one-pot hydrothermal reaction. Through optimizing of the g-C3N4 component proportion, Zn0.5Cd0.5S heterostructure decorated with 3 wt% g-C3N4 nanosheets (Zn0.5Cd0.5S/g-C3N4) displayed a high H2-production rate of 4.956 mmol g−1h−1, which was 2.73 times higher than that of pure Zn0.5Cd0.5S nanoparticles. Additionally, when 1 wt% RGO was further introduced to the binary Zn0.5Cd0.5S/g-C3N4 heterostructure, the obtained ternary Zn0.5Cd0.5S (0D)/g-C3N4 (2D)/RGO (2D) composite catalyst exhibited the highest photocatalytic H2-production activity, whose H2-production rate was 5.41 times higher than that of pure Zn0.5Cd0.5S nanoparticles. The excellent activity of the ternary Zn0.5Cd0.5S/g-C3N4/RGO catalyst is associated with the cooperative effects of Zn0.5Cd0.5S nanoparticles, RGO and g-C3N4 nanosheets, which not only extend the light absorption range and intensity, but also reduce the overpotential for H2-production and dramatically improve the charge mobility and separation, thus boosting the H2-evolution rate over the ternary composite photocatalyst.