Due to the special layered structure and appropriate band gap, BiOX (X= Cl, Br, I) shows good photocatalytic activity and stability in visible light. Changing the composition of halogen in BiOX_s can adjust the band gap, thereby optimizing the photocatalytic performance. In this paper, a series of BiOCl_xBr_yI_z(x, y, z=0.1, 0.2, 0.33, 0.6, or 0.8) composite catalysts were prepared by a mixed solvothermal method, and their structural morphologies and optical properties were characterized. Furthermore, the photocatalytic performance of these catalysts was investigated by degrading 15 mg/L methyl orange (MO) under a 300 W xenon lamp with a 400 nm cutoff filter for 180 min. The results show that BiOCl_xBr_yI_z composite catalysts displayed a microsphere structure assembled by nanosheets and their degradation efficiencies towards MO were higher than those of pure BiOX (X= Cl, Br, I) except BiOCl_0.8Br_0.1I_0.1. Among the prepared BiOCl_xBr_yI_z composite catalysts, BiOCl_0.33Br_0.33I_0.33 composite catalyst exhibits the best degradation performance, its degradation efficiency can reach 98.4%. The reason may be that the composite catalyst with a more suitable band gap can make better use of visible light and improve the separation and transmission efficiency of photogenerated carriers.