Abstract: With the development of the aerospace industry and the need for industrialized production, the casting processes of large diameter aluminum alloy ingot have come into focus in the industry. Among them, ultrasonic-assisted casting technology is widely used. Ultrasonic-assisted casting technology has the advantages of improving solute segregation of ingot and refining solidification organization. Other advantages have been widely reported. At present, most of the aluminum ingots used in the non-hot top ultrasonic casting process with very shallow liquid cavities, while the casting process does not involve the issue of ultrasonic vibration depth. With the use of a hot-top mold for ultrasound in the casting and casting process of large diameter ingot, the liquid level of aluminum melt is very high. The ultrasonic vibration depth will affect the cavitation range and finally affect the fine grain effect of the ingot. In the present study, a double source ultrasonic vibration system was applied in the process of semi-continuous casting of aluminum alloy with a diameter of 650 mm, and the influence of ultrasonic immersion depth on the macroscopic solidification structure of ingot was studied. Based on the test results of the solidified microstructure of aluminum alloy ingot and the simulation results of the sound field of the finite element software such as ANSYS, the mechanism of the microstructure refinement of the aluminum alloy ingot under different vibration depths was discussed at length. Study results show that, with increasing vibrational depth of the supersonic radiation rod, the whole cross section of the ingot is further refined, and grain shape changs from developed dendrites to equiaxed dendrites. Because of the end faces of the ultrasonic radiation rod, there is a vibrational peak at the fixed position, which leads to different ultrasonic cavities under different ultrasonic vibrational depths in the aluminum melt. This leads to different refinement mechanisms of the solidified structure.