A new type of light stabilized nanocatalyst containing different proportions of “flower-like” BiOCl nanosheets and g-C3N4 were synthesized through the traditional hydrothermal method to control the growth process. The superior morphology features of BiOCl/g-C3N4 photocatalysts were explained using SEM, EDS, TEM, XRD, and BET. The prepared BiOCl/g-C3N4 nano photocatalyst was proved by a series of experiments, and the photochemical catalytic performance of BiOCl/g-C3N4 was studied through the degradation reaction of the colorless antibiotic metronidazole. TOC and GC–MS analysis were used to discover the products of metronidazole after photodegradation. Many experiments suggested that BiOCl/g-C3N4 has a very excellent photochemical catalytic performance exposed to visible light irradiation. During the DRS, ESR, and trapping experiment proving, the interaction between BiOCl and g-C3N4 will improve the separation of photo-generated holes and electrons, thus greatly enhancing the photostability and photocatalytic performance of the material.