Abstract: The ingot of AZ31B magnesium alloy was prepared by casting in ultrasonic field.The effect of ultrasonic wave on solidified structure and properties of the alloy after rolling was studied.The results show that compared with the ingot casted without ultrasonic field the grain size of the AZ31B magnesium alloy ingot after casting in ultrasonic field could reduce from 250-350 μm to 120 μm.The ultrasonic vibration promoted the precipitation of the second phase which strengthened the base and improved the strength of the material.The tensile strength and yield strength were improved by 18.6% and 25.8% respectively after rolling the magnesium plate to 30 mm,and the elongation value had no significant change.