Zero-dimensional (OD) quantum dots (QDs)/two-dimensional (2D) nanosheets heterojunctions have attracted significant attention due to their high charge mobility and effective charge carrier separation. Herein, a novel MoS2 QDs/graphitic carbon nitride (MoS2 QDs/g-C3N4) heterojunction composite with multiple unique advantages over the traditional MoS2 nanoparticles/g-C3N4 composites has been prepared by a facile polymerization method. The obtained MoS2 QDs/g-C3N4 composite exhibits superior visible-light-driven photocatalytic performance toward the photodegradation of organic pollutants and possesses a different catalytic degradation mechanism compared with the pure g-C3N4. The radical species trapping experiments and ESR measurements indicate that the (OH)-O-center dot radical is one of the major active species generated by the MoS2 QDs/g-C3N4 composite, whereas the (OH)-O-center dot radical only plays a minor role in the photodegradation processes catalyzed by pure g-C3N4. A type-II staggered band alignment is observed in the MoS2 QDs/g-C3N4 composite, which accounts for its efficient separation of photo-induced charge carriers and formation of (OH)-O-center dot radicals. The superior visible-lightdriven photocatalytic performance could be attributed to the strong coupling and band alignment between the MoS2 QDs and g-C3N4 nanosheets, leading to an enhanced efficiency for the generation and separation of the photo-induced charge carriers. This study provides new insights into the design and fabrication of novel g-C3N4 based hybrid photocatalysts with reasonable electronic structures for photochemical reactions.