Mg-4Al-1Si magnesium alloy melt was treated by self-designed ultrasonic treatment equipment for magnesium alloy melt. The effects of ultrasonic power, treating time and treating temperature on solidification structures of Mg-4Al-1Si magnesium alloy were investigated. The results show that ultrasonic treatment can markedly refine the grain size of Mg-4Al-1Si magnesium alloy, and the grain size is improved by enhancing ultrasonic power, elevating treating temperature, or prolonging treating time. The grain size of Mg-4Al-1Si magnesium alloy treated by 270 W ultrasonic power for 50 s at 650 °C is very fine, 107 μm, which is only 28% as that without ultrasonic treatment (383μm). In addition, the morphology of Mg2Si in Mg-4Al-1Si alloy with ultrasonic treatment changes from coarse Chinese character shape without ultrasonic treatment into tiny granulation with evenly distribution in the matrix.