Abstract: BiOI is considered as one of the most promising visible light photocatalysts because of its narrow band gap and layered structure. BiOI photocatalyst was synthesized by hydro-solvo-thermal process using bismuth nitrate and potassium iodide as the raw materials, and a mixed solution of water and 2-methoxyethanol was used as the solvent. The crystal structure, micro-morphology, light absorption performance and specific surface area of the samples were characterized by XRD, SEM, UV-Vis-DRS and BET, respectively. The photocatalytic activities of BiOI samples under visible-light irradiation were evaluated by the degradation of methyl orange. The results showed that the volume ratio of 2-methoxyethanol in the mixture solvent had a significant influence on the morphology and performance of the BiOI. The morphology of the BiOI photocatalyst prepared at 50% volume ratio of 2-methoxyethanol in the mixture solvent was a flower microsphere. The BiOI microspheres were fabricated by nanosheets with the characteristics of mesoporous structure. Because of the addition of 2-methoxyethanol, the crystal growth of BiOI was strongly restrained, and the dominant growth along (110) plane was very obvious. The photocatalytic activity of BiOI had been significantly improved because of the larger specific surface area and the flower spherical structure. After 150 min of visible light irradiation, 77.9% methyl orange were degraded, and the degradation rate was 14 times that of the nanosheet BiOI photocatalyst. The photocatalytic mechanism of BiOI under visible light was proposed. The photoinduced holes(h⁺) and ∙O₂⁻ are the active species in the process of photocatalytic degradation of MO. Furthermore, the photo-generated holes (h⁺) were the most important photocatalytic active species.