Metal-halide perovskite quantum dots (QDs) have emerged as a potential photocatalyst owing to their remarkable optoelectronic properties. However, the poor stability and insufficient charge transportation efficiency of this type of materials have hindered their applications in the photocatalysis field. Herein, we decorated CsPbBr3 QDs on two-dimensional (2D) ultrathin g-C3N4 (UCN) nanosheets to develop a 0D/2D CsPbBr3/UCN composite photocatalyst. The introduction of UCN can not only improve the stability of CsPbBr3 QDs by passivating the surface ligands of CsPbBr3 QDs, but also facilitate the charge transfer due to the suited band gap alignment. Consequently, the obtained CsPbBr3/UCN heterostructure exhibited superior photocatalytic performance to both pristine CsPbBr3 QDs and UCN. This work has provided an efficient strategy for the design of CsPbX3-based heterostructure with high stability and photocatalytic activity.