The invention belongs to the field of light 
alloy machining, and particularly relates to a variable frequency ultrasonic semi-
continuous casting method for 
magnesium alloy. The method comprises the steps of 
alloy smelting, melt delivery and consolidation forming under ultrasonic treatment. 
Magnesium alloy melt is treated in a real-
time variable frequency ultrasonic field introduced in the semi-
continuous casting process, and the quasi real-time tracking of the resonant frequency of 
metal melt is achieved; and liquid 
cave melt is subjected to the ultrasonic 
resonance treatment continuously in the whole semi-
continuous casting process, so that the continuous ultrasonic high-efficiency treatment of the 
magnesium alloy melt is achieved in the solidification process. In the method provided by the invention, an 
ultrasonic radiation rod is extended into the 
magnesium alloy melt in the 
magnesium alloy semi-continuous 
casting process, so that the 
cavitation effect and 
acoustic streaming effect produced by the variable frequency ultrasonic field directly act on the 
magnesium alloy melt solidifying. With the method provided by the invention, the 
magnesium alloy solidification behavior can be changed obviously, the structure is refined, and the uniformity of the billet solidification structure as well as the 
mechanical property of magnesium alloy billets are improved; and the billet 
casting defects are reduced obviously, and the quality of the magnesium alloy billets is improved greatly.