The invention provides a 
magnesium alloy bar power ultrasonic semi-
continuous casting and flaw detection device and method. The 
magnesium alloy bar power ultrasonic semi-
continuous casting and flaw detection device comprises a power ultrasonic generating device, an ultrasonic flaw detection 
processing system and a vertical direct water-cooling semi-
continuous casting machine. The power ultrasonic generating device comprises an 
ultrasonic generator, an ultrasonic time-sharing 
vibration control system and three ultrasonic transmitting assemblies with the same specification. Each ultrasonic transmitting 
assembly comprises an ultrasonic 
transducer, an ultrasonic amplitude 
transformer and a tool head. The ultrasonic flaw detection 
processing system comprises an ultrasonic receiving sensor, a 
signal processing and determination module and an ultrasonic flaw detection display screen. The ultrasonic transmitting assemblies are inserted into molten 
magnesium alloy at the same time, and controlled by the ultrasonic time-sharing 
vibration control system to transmit power 
ultrasound at intervals, and the molten 
magnesium alloy solidifies under the action of an ultrasonic field so that fine-grain cast bars can be obtained. Ultrasonic 
waves transmitted by the ultrasonic transmitting assemblies serve as a 
signal source, the waveforms of the ultrasonic 
waves are processed through the ultrasonic receiving sensor and the 
signal processing and determination module, and the internal flaw conditions of the cast bars are determined according to prompts on the ultrasonic flaw detection display screen.