To facilitate the realize high-value utilization of coal tar pitch, carbon quantum dots (CQDs) were synthesized using ball-milling-assisted H₂O₂ oxidation method with tar pitch as carbon source. Subsequently, Ag₃PO₄/g-C₃N₄/CQDs composites were prepared through the electrostatic self-attraction of CQDs, Ag₃PO₄ and g-C₃N₄. These composites were then employed for the photocatalytic degradation of dyes, including methyl orange (MO), methylene blue (MB) and rhodamine B (RhB). A case study of MO, the degradation rate constant of Ag₃PO₄/g-C₃N₄/CQDs reached 0.02469 min⁻¹, which was 11 times and 1.6 times higher than that of g-C₃N₄ and Ag₃PO₄. It found that Ag₃PO₄:g-C₃N₄ mass ratio of 1:1 and 5 mg/L CQDs solution loading were optimal. Characterization analysis revealed the specific surface area of the catalyst was increased and optimized band gap structure after the combination of CQDs as the co-catalyst. The possible reaction mechanism was also expounded, Ag₃PO₄/g-C₃N₄/CQDs constructed Z-scheme heterostructure, with CQDs as electron bridge. In addition, the impact of catalyst mass, initial concentration, and pH on the degradation process was studied, as well as the cyclic stability of the catalyst. Based on capture and ESR tests, the main active substances involved in the reaction were in the following order: h⁺ > ·OH > ·O₂⁻. These experiments provided a new idea on the application of coal-based CQDs in the field of visible light catalytic degradation.