Semisolid A356 alloy, which is grain-refined by Pr, was prepared by low superheat pouring. The effects of grain-refined by Pr on the morphology and the grain size of the primary α-Al in semisolid A356 alloy are researched. The results indicate that semisolid A356 alloy with particle-like and rosette-like primary α-Al can be prepared by low superheat pouring from liquid A356 alloy grain-refined by Pr. The grain size and particle morphology of primary α-Al in A356 alloy are markedly improved by 0.3wt% Pr. The refining mechanism of Pr on the morphology and the grain size of the primary α-Al in semisolid A356 alloy were delved.