Abstract: The mechanism of the grain crystallization in TIG weld pool under different ultrasonic action is of great significance for optimizing the ultrasonic assisted TIG welding process. The grain distribution characteristics in the weld of aluminum-lithium alloy is analyzed by changing the ultrasonic action stage and style.The results showed that the edge of the weld is the broken columnar grain and the center of the weld is equiaxed grain with the ultrasonic action imposed after the arc is extinguished. The grain is equiaxed grain when the ultrasonic action is acted during whole welding process. Continuous ultrasonic action and transient ultrasonic action have different effects on crystal crystallization. Continuous ultrasonic action mainly promotes heterogeneous nucleation of grains in weld pool for the formation of equiaxed crystal. Transient ultrasonic action mainly broken dendrites. Ultrasonic refinement mechanism of the grain structure in the TIG weld of Al-Li alloy is directly related to the stage of ultrasonic action.