There is an obvious difference in solidification characteristic between low superheat pouring low density electromagnetism stirring.In the general solidification theory,the full principle of nucleation and growth of stirring alloys is important to understand the evolvement and formation of semi-solid structure deeply.In this paper,the effect of stirring on alloy melt and nucleation and the inhibitory effect of stirring on dendrite formation in mould wall of Al-30%Si alloy are analyzed,which is to supply base for setting up systemically the solidification theory of low superheat pouring low density electromagnetism stirring.