Abstract: The light absorption and photo-induced carrier recombination of g-C₃N₄ are the key problems that limit its efficient photocatalytic applications. Herein, carbon dots (CDs) were synthesized using coal pitch as precursor and then the CDs/g-C₃N₄ composite catalyst was prepared by ultrasonic-assisted method. The structure, optical and photoelectrochemical properties of the catalyst were characterized by TEM, XRD, UV-Vis diffuse reflection spectrum, PL spectrum, EIS test and photocurrent response test. The results show that the regulation of band structure and the formation of interface after the introduction of CDs expand the range of light absorption of the composite photocatalyst, promote the effective separation and migration of photogenerated electrons and holes, and thus facilitate the photocatalytic reaction. Using rhodamine B (RhB) as the model, the photocatalytic activity of CDs/g-C₃N₄ composite catalyst is significantly higher than that of pure g-C₃N₄ under visible light irradiation. The degradation rate of RhB can reach 98.6% within 40 min, and the degradation rate constant of CDs/g-C₃N₄ is 6.8 times that of g-C₃N₄. The capture of experiment of active species reveals that •O₂⁻ plays a major role in the degradation system. In addition, CDs/g-C₃N₄ composite catalyst exhibit good stability. After 5 cycles, the degradation rate of RhB is still up to 97.5%, which shows a good application prospect in visible light photocatalysis.