BixOyBrz photocatalysts exhibit great potential in the application of organic pharmaceutical wastewater treatment and air purification. However, the catalysis activity and application of photocatalyst is greatly limited by its intrinsic high recombination rate of photo-generated charge carriers. In this work, a novel Bi3O4Br/Bi12O17Br2composite photocatalyst was prepared via a facile in-situ hydrolysis-calcination route and its photocatalytic performance was evaluated by the sulfamethoxazole (SMX) degradation efficiency via simulated solar light irradiation. The structure and property were analyzed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), nitrogen adsorption and desorption, UV-Vis diffuse reflectance spectra (UV-Vis DRS), electrochemical impedance spectroscopy (EIS), Steady photoluminescence and Mott-Schottky curves. Results reveal that Bi3O4Br/Bi12O17Br2 composite photocatalyst display the optimum photocatalytic performance, and its removal efficiency of SMX reaches 87% under 30 min simulated solar light irradiation, which is improved approximately 30% and 24% than that of pure Bi3O4Br and pure Bi12O17Br2, respectively. Based on characterization analysis, the superior performance is attributed to higher electron-hole separation rate and lower charge transfer resistance. Finally, the underlying photocatalytic mechanism was elucidated based on the band structure and radical scavenging experiments. This findings provide novel ideas and methods of the construction of the composite photocatalysts system for strong organics pharmaceutical wastewater treatment.