Bi2O2CO3/Bi4Ti3O12 composite heterojunction photocatalyst was prepared using in-situ generation method. The crystal structure, microstructure, elemental composition, and optoelectronic properties of the prepared material were studied through a series of characterization methods. The degradation activity of the prepared photocatalyst was studied by degrading the antibiotic ofloxacin under visible light. The experimental results show that the degradation activity of the composite photocatalyst is superior to that of a single component photocatalyst, and the optimal composite sample BTC-2 has an apparent rate constant that is 6.3 times and 2.2 times that of pure Bi2O2CO3 and Bi4Ti3O12, respectively. The 4 cycles of experiments indicate that the composite sample has good stability. According to the electrochemical test results, it was found that the BOC/BTO composite photocatalyst has good photo generated carrier separation efficiency and excellent charge transfer efficiency, thus exhibiting good degradation activity towards ofloxacin. The free radical capture experiment indicates that the main active species of BOC/BTO composites in photocatalytic experiments are holes and superoxide radicals. In addition, charge transfer pathways and mechanisms for enhancing activity were proposed through free radical capture experiments and band theory.