In this paper, semi-solid slurry was prepared by power ultrasonic on flowing A356 melts. A simple device of ultrasonic treatment was made and the method of applying power ultrasound from the top of ingot was adopted. we investigated the influence of the experimental parameters impact on the size and morphology of the primary phase α-Al in A356 slurry, which included the power of the power ultrasound, treatment temperature and the distance between ultrasonic source and the melts ( here refer to from vibration head to the bottom of melt channel in this paper). The regularity of ultrasonic influence on grain refinement and morphology spheroidizing of primary phase α-Al in A356 alloy were obtained. The results show that when the loading power range from 800W to 1000W, the grain refinement effect goes more obviously with the increasing power of ultrasonic, howerer, when the power is up to 1500W, the effect goes worse; and the best primary phase is obtained at 615 oC among the four treatment temperature (645 °C, 630 °C, 615 °C, 600 °C); with the increase of the distance, among the four groups data (7mm, 9mm, 12mm, 15mm), the microstructure of A356 alloy goes more coarse and the segregation is more serious, the average grain size and form factor of the best primary phase α-Al was 29.47um and 0.78, respectively in this study.