It is still a big challenge to reveal the correlation between crystal structure and properties. In this work, bismuth oxychloride ((BiO)(2)OHCl) is prepared by a simple precipitation method, and the effect of mixed anion layer on the photochemical properties is mainly investigated. The sample is characterized by polycrystalline X-ray powder diffraction (XRD), scanning electron microscope (SEM), infrared spectroscopy (IR), high-resolution transmission electron microscope (HRTEM), selection area electron diffusion (SAED), UV-visible diffuse reflectance spectra (UV-DRS), photoluminescence fluorescence (PL), etc. It is found that under ultraviolet light irradiation (lambda <= 400 nm), the photoactivity of (BiO)(2)OHCl is 1.8 times higher than that of BiOCl for the degradation of MO, although (BiO)(2)OHCl has a much lower BET area (3.4 m(2) g(-1)) than BiOCl (15.9 m(2) g(-1)). Comparing (BiO)(2)OHCl with BiOCl, parts of Cl in the anion layer of BiOCl are replaced by hydroxyl. The mixed anion layer has shifted the energy bands down, compared with BiOCl. Because hydroxyl has a higher ability to attract electron than Cl, the insertion of OH- has greatly increased the electron density of the anion layer of (BiO)(2)OHCl. Thus, the internal electric field (IEF) of (BiO)(2)OHCl has been strengthened, which is firmly demonstrated by the photocurrent results. The strengthened IEF could favor for the separation and transfer of photogenerated charges, leading to an improved photoactivity. This study provides a new idea to develop the efficient photocatalysts through building mixed anion layer.