Abstract: This paper studies the transform course for primary phase morphology of the semi-solid A356 alloy billet, produced by low superheat pouring during reheating in solid-liquid phase region. The results indicate that the primary phase of semi-solid A356 alloy is gradually spheroidized during the holding in solid-liquid phase region, in which spheroidization is speeded up with the increase of the holding temperature. There is little effect of holding temperature and holding time on grain roundness of semi-solid A356 alloy, but the grain size of the alloy increases with increasing of the time and the temperature. The optimizing reheating technology of semi-solid A356 alloy is that the billit is held 30 min at 583℃, in which the average equal-area-circle grain diameter is 80μm, and the average grain roundness is 0.83.