Abstract: Ultrasonic field was applied to prepare 7050 aluminum alloy semi-casting aluminum alloy ingot, the microstructure of the ingots were observed by Leica optical microscopy and the effect mechanism of ultrasonic field on hot tearing forming was analysed by combining the RDG hot tearing criterion and solidification shrinkage theory. The result show that a crack was found in the center of the ingot without ultrasonic field and no crack was found in the ingot with ultrasonic field. The microstructure of ingot with ultrasonic field was homogeneous, the grain size was obviously refined and the microstruure was changed into equiaxed grains from dendritic crystals without ultrasonic field. The ultrasonic field could effectively depress hot tear forming.