Abstract: The low visible light activity, low photogenerated carrier mobility and easy recombination of ZnO limit its practical application in the field of photocatalysis. To overcome these limitations, a carbon quantum dots (CQDs)-reduced graphene oxide (rGO)/ZnO ternary composite catalyst was prepared by an ultrasonic-assisted impregnation method and characterized for its crystal structure, morphology, and photoelectric properties using XRD, SEM, TEM, X-ray energy dispersive spectrometry (XEDS), XPS, BET, UV-Vis DRS, valence band (VB)-XPS, photoluminescence (PL), transient photocurrent response (TPR), and electrochemical impedance spectroscopy (EIS). The photocatalytic activity of CQDs-rGO/ZnO composite catalyst was investigated with simulated antibiotic wastewater metronidazole (MTZ) as degradation object. The results show that the introduction of rGO and CQDs can optimize the band structure of ZnO and enhance its absorption of visible light. The strong electrical conductivity of rGO and CQDs can promote the rapid transfer and separation of photogenerated carriers, and effectively improve the photocatalytic activity of CQDs-rGO/ZnO composite catalysts. The quenching experiments show that hydroxyl radical (•OH) and superoxide radical (\text•\rmO_2^ - ) are the main active substances in the reaction process. When the recombination amount of CQDs is 1%, the photocatalytic activity of CQDs-rGO/ZnO is the best, and the degradation rate of MTZ can reach 87.8% under visible light irradiation for 2.5 h. After four cycles, 75.2% of MTZ can still be degraded, indicating that the physicochemical properties of the CQDs-rGO/ZnO composite catalyst are stable.