In order to reveal the internal mechanism of ultrasonic-assisted freezing and clarify heat and mass transfer law and component migration characteristics during the freezing under the action of sound, the cavitation and thermal effects of the ultrasonic wave were analyzed according to the sound field theory. Based on the conservation of energy and mass, a mathematical model of droplet freezing and salt migration in the effect of the ultrasonic wave was established, which presented the heat and mass transfer and the salt migration law. The model can reflect the effect of the sound field on the freezing and was used to analyze the phase change freezing process, which involved the ultrasonic cavitation effect and the state change of bubbles. In different conditions, some important parameters, such as temperature variation, liquid ratio, concentration ratio, and salinity were studied, which can provide the path to improve the freezing efficiency and the brine separation effect. The results showed that ultrasound was helpful for heat and mass transfer. The dropping temperature drops rapidly, ultrasound was conducive to the freezing of drops. It was that the low-frequency ultrasound will increase the growth cycle of bubbles in droplets and the maximum diameter of bubbles, to strengthen heat and mass transfer. The higher the salt concentration was, the slower the freezing process was. It was that the smaller the diameter, the greater the mass transfer and heat dissipation caused by the cavitation effect. When the droplet diameter was 2 mm, the time to reach the freezing point was 15 s when the salt concentration was 5wt%, and 20 s when the salt concentration was 8wt%. When the concentration of salt in the droplet was low, the change range of salt migration became more intense. It was that the lower the concentration of droplets, the stronger the driving force of salt migration potential difference, and the range of salt migration was becoming more and more intense.