A 3D spherical bismuth oxyiodide (BiOI) photocatalyst was prepared by solvothermal method using copper nitrate pentahydrate and potassium iodide as raw materials , ethylene glycol as solvent, and the morphology of BiOI was controlled by hydrothermal temperature. The material was characterized by SEM, EDS, XRD and UV-vis. BiOI and persulfate (PMS) were coupled to degrade azo dye orange II under visible light. The effects of BiOI dosage, PMS dosage, orange Ⅱ concentration, pH and common anions on the degradation system were investigated. The results showed that BiOI had a good visible light response with a band gap of 1.8eV. The degradation efficiency reached 97.0% when BiOI dosage was 0.2g/L, PMS concentration was 0.2mM/L and orange Ⅱ concentration was 100mg/L, and the degradation process was in accordance with the quasi-second-order kinetic model. BiOI is excited by visible light to generate photogenerated carriers. On the one hand, it reacts with PMS to generate ·SO4-, ·OH, on the other hand, it reacts with dissolved oxygen in the system to generate ·O2-, ·O2- reacts with H+ in H2O to generate 1O2. In addition, PMS self decomposes to produce 1O2. The degradation process is a multi-channel heterogeneous reaction, and ·O2- and 1O2 are the main free radicals in the degradation process.