The dietary fiber in quinoa has excellent physicochemical properties and functional activities, and is considered a high-quality dietary fiber. This study focused on the quinoa bran soluble dietary fiber (QBSDF), which was modified by ultrasonic treatment with different powers (0,200, 400,600, 800, 1000W). The particle size, molecular weight, porosity, specific surface area, Zeta-potential, monosaccharide composition, infrared spectrum, microstructure, water-holding capacity, oil-holding capacity, glucose adsorption capacity, and viscosity were determined before and after modification to explore the effect of ultrasonic treatment on the physicochemical and structural properties of QBSDF. The results showed that the water-holding capacity, oil-holding capacity, and glucose adsorption capacity of QBSDF were significantly improved after ultrasonic modification (P<0.05). The analysis of rheological properties showed that the viscosity value and storage modulus of QBSDF increased significantly and the loss modulus decreased after ultrasonic modification. After ultrasonic modification, the molecular weight and average particle size of QBSDF were significantly reduced (P<0.05), and the surface structure became looser and showed honeycomb-like porous characteristics. The porosity and specific surface area of QBSDF increased significantly (P<0.05), and showed an upward trend with the increase of ultrasonic power. No changes occurred in the monosaccharides and chemical group types, but the composition ratio of monosaccharides and the peak intensity of chemical groups changed significantly, indicating that ultrasonic modification disrupted the chemical bonds related to —OH, C—H and other groups in dietary fiber. Therefore, ultrasonic treatment could significantly change the microstructure of QBSDF, increase its contact area and binding sites with substances, and improve its physicochemical properties, such as water-holding capacity, oil-holding capacity and glucose adsorption capacity. In addition, it was also found that the water-holding capacity and oil-holding capacity of QBSDF were higher than those of oat dietary fiber after ultrasound modification. Therefore, ultrasonic treatment could be used as an excellent method to modify the physicochemical properties of QBSDF, providing a theoretical basis for improving the utilization of quinoa dietary fiber and further exploring its functional value.