In this study, Co3O4 quantum dots decorated phosphorus-doped g-C3N4 nanosheets was synthesized though one-step thermal polymerization method using vitamin B12 (VB12) as precursor of cobalt and phosphorus mixed with melamine. The prepared photocatalysts were characterized by X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR), BET, transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (DRS), photoluminescence (PL) and electrochemical measurements. Appropriate amount of introduced VB12 can improve photocatalytic activities while excess caused inhibition. The sample with the weight ratio of VB12 in precursors at 1% exhibited the optimum photodegradation efficiency for metronidazole (MTZ), about 2 times as higher as the removal rate of pure g-C3N4. Several effects on the photocatalytic activity was also investigated and the results showed that enough dissolved oxygen and presence of Fe3+ was found to promote MTZ photodegradation, while NO3-, Ca2+ and HCO3- would inhibit it. Superoxide radical center dot O-2(-) was identified as the dominate active specie in photodegradation process and possible enhancement mechanism of introduced VB12 for photocatalytic activities was proposed. It was inferred that the synergistic effect of generated Co3O4 quantum dots and P-doped g-C3N4, including increasing the separation efficiency of photo-inducted electrons and holes, inhibiting the recombination process of them and decreasing the band gap energy, could be the reason for the better photocatalytic activity. At last, MTZ degradation pathway in the photosystem was determined.