Abstract: High energy ultrasonic instrument was used to prepare as-cast A356 alloy.The effect of high-energy ultrasonic on the microstructure and mechanical properties of A356 alloy was investigated by means of SEM and universal testing machine.The mechanism was analyzed.The results indicate that the morphology and sizes of the α-Al and Si phases were obviously improved after ultrasonic treatment,the Si phase with long dendritic-shape was refined,and α-Al phase was refined and circled.The best ultrasonic effects were obtained with 1.2 kW power and 600 s applying time.When the applying time was fixed,the values of the tensile strength,yield stress and elongation of the A356 alloy increased with the increasing ultrasonic power,the maximums could reach 294.37,207.98 MPa and 6.85%,respectively,which were 1.56,1.93 and 1.10 times to that of the alloy without high-energy ultrasonic treating.With the increasing applying time,the tensile strength,yield stress and elongation of the A356 alloy increased first,and then decreased.