Dynamical process of iodine release in potassium iodide solution has been investigated under the irradiations of ultrasound and visible light respectively and simultaneously. Under ultrasonic irradiation, the efficiency of iodine release increases almost linearly with the irradiation time, indicating the zero-order reaction. On the other hand, the efficiency of iodine release increases exponentially with the irradiation time under visible light irradiation and combined irradiation of ultrasound and visible light, suggesting the pseudo first-order kinetics. We further find a significant synergistic effect induced by combined irradiation of ultrasound and visible light, which may be due to the well-mixed condition in the photochemical reactor with the applying simultaneous ultrasound. We further investigate influence of the initial concentration of potassium iodide on the synergistic effect.