The semi-solid A356 alloy slurry is prepared by compound process, and the effect of the compound process on morphology and size of primary α-Al in A356 alloy is researched. The results indicate that the compound process remarkably affects the morphology and the size of primary α-Al. Primary α-Al with particle-like is distributed uniformly in A356 alloy, and there is no the transient area of change in structure morphology. Compared with the samples prepared by low superheat pouring and slightly electromagnetic stirring, the nucleation rate, morphology and grain size of primary α-Al in A356 alloy are markedly improved by the compound process. The mechanism of refining grain in the compound process is probed.