Constructing efficient and recyclable photocatalysts is a promising method to solve antibiotic pollution. Herein, we designed and fabricated C3N5/TiO2 S-scheme heterojunction film on flexible Ti foils through the self-absorbed. The C3N5 nanosheet was tightly anchored on the TiO2 arrays with an intimate TiO2-C3N5 interface. The heterojunction exhibited an excellent photocatalytic performance and the reaction rate constant of photocatalytic degradation gatifloxacin (GAT) over C3N5/TiO2 up to 0.0252 min−1, which was 1.85 folds that of pure TiO2 arrays. The outstanding photocatalytic performance was ascribed to high-efficiency transfer and separation of photoinduced carriers and highly hydrophilic surfaces. The removal rate of GAT under different parameters suggested that the as-prepared C3N5/TiO2 has high adaptability. Moreover, the three possible GAT degradation pathways over C3N5/TiO2 were proposed based on the results of the liquid chromatograph-mass spectrometer (LC-MS) and density functional theory (DFT). Finally, a reasonable S-scheme heterojunction was proposed based on the results of experiments and DFT calculation. This work may help in designing an S-scheme heterojunction for the degradation of antibiotic pollutants.