Based on the analysis of the characteristics of hypoeutectic Al-Si alloys melt structure and physical properties of Al,Si,Ti and Sr,effects of refinement and modification on melt structure of the alloy have been investigated with electron theory.In hypoeutectic Al-Si alloy melt,refinement and modification can be described to the interaction of Al with Ti and Si with Sr,respectively,which can waken heterogeneity of melt structure,and not only improve melt structure,but also restrain the fading of each other.