In this work, uniform BiOCl microspheres were used as self-sacrificed template to construct Bi2O3/BiOCl heterojunctions by a simple alkaline treatment method. Details of structure and chemical properties were carefully characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM, X-ray photoelectron spectroscopy, UV-vis diffuse reflectance spectroscopy, and Brunauer-Emmett-Teller techniques. The results indicated that BiOCl microspheres composed of numerous nanosheets transformed to rod-like Bi2O3 nanostructures during the alkaline treatment, which resulted in strong visible-light absorption. The as-obtained Bi2O3/BiOCl heterojunctions exhibited remarkably enhanced photocatalytic performance for the degradation of methyl orange (MO) under visible-light irradiation, which was similar to 1.2 and 2.5 times as that of pure BiOCl and Bi2O3, respectively. The formation of Bi2O3/BiOCl heterojunctions facilitated the transfer and separation of photogenerated charge carriers which was further confirmed by the photocurrent measurement and photoluminescence spectra results. The finding reported here offers a valid in situ route for constructing heterojunction photocatalysts to effectively decompose the organic pollutants in aqueous solution.