Abstract: The effects of ultrasonic treatment temperature and time on the size and morphology of the primary phase of Al Si7Mg alloy were systematically investigated to reveal the ability of ultrasonic treatment to control the microstructure of aluminum alloy under different temperature and treatment time conditions. Three typical temperature conditions were selected for the experiments: 615 ℃(low semi-solid temperature), 625 ℃(high semi-solid temperature) and 705 ℃(die-casting temperature), and ultrasonic treatment time ranging from 15 s to 90 s was applied. The results show that the ultrasonic treatment can significantly affect the microstructure of the alloy, especially at 625 ℃, the best ultrasonic treatment effect is achieved, with the average size of α-Al grains reducing to 57 μm and the grain roundness ranging from 0.72 to 0.80. In addition, the ultrasonic treatment time has a significant effect on the grain refinement, especially in the treatment time of 15-30 s, the α-Al primary phase is significantly refined, and the morphology transforms from dendritic structure to non-dendritic structure. The mechanism of grain refinement in the semi-solid temperature range is mainly related to the fragmentation and refinement of dendritic arms induced by ultrasonic treatment. Compared with the traditional casting methods, ultrasonic treatment can not only optimize the microstructure of aluminum alloy, but also enhance the mechanical and processing properties of the alloy by controlling the morphology of the primary phases. It is especially promising for the manufacturing of high-strength and lightweight aluminum alloys with a wide range of applications.