Graphitic carbon nitride (g-C3N4, GCN) shows excellent photocatalytic activity for a myriad of reactions due to its unique traits and semiconducting properties. The design of a semiconductor heterojunction by hybridizing GCN and other materials with appropriate band structures has profiled one of the most fascinating approaches to enhance the photocatalytic efficiency of GCN. In our simulation, a metal-free heterojunction was developed by incorporating zero-dimensional (0D) black phosphorus quantum dots (BPQDs) with two-dimensional (2D) GCN. The 0D/2D BPQD/GCN heterojunction was systematically investigated by using density functional theory (DFT) calculations. Various orientations of BPQD on GCN were compared. The electronic structure and charge density distribution of the BPQD/GCN composite were calculated to examine the most favorable configuration. Furthermore, the charge separation and transfer mechanism of this heterojunction structure were discussed from the perspective of computation. Our study reveals that BPQDs and GCN formed a Type II heterojunction with a high stability and robust photocatalytic efficiency. Overall, the present work not only elucidates theoretical guidance for taking the merits of BPQDs and GCN, but also paves a new frontier for engineering metal-free 0D/2D heterojunction nanocomposite systems.