In this work, the azo dye Congo red (CR) was degraded by a Fe–In2O3 catalyst under the irradiation of ultrasonic. The Fe–In2O3 catalyst was prepared by a fast and moderate solvothermal method followed by the characterization of X-ray diffraction and scanning electron microscope. The effects of operating parameters, such as catalyst composition, catalyst dosage, initial dye concentration, ultrasonic power and ultrasonic frequency on degradation process were discussed. In the experiment, the optimum CR removal of 97.75% in 60min was achieved under the conditions, i.e., catalyst dosage of 0.06g⋅L−1, CR concentration of 10mg⋅L−1, ultrasonic frequency of 45kHz and ultrasonic power of 100W. Besides, the CR degradation behavior by the catalyst with ultrasonic is well in accordance with the first-order kinetic model.