In this paper, high Mg addition (2wt%, 4wt%, 6wt%) to AlSi7 alloy is researched. The results show that Mg can react with Al2O3 and Si element, the MgAl2O4 phase which has a good wettability with Al melt and Mg2Si phase are formed, both of them can uniformly distribute in the alloy and induce a particle based stabilization mechanism, which can significantly reduce the negative impact of gravity drainage and capillarity. High Mg content can also change the pores formation in early stage, pores nucleate in a pool of liquid and grow in a fairly spherical way, so high expansion rate and good pore structures can be obtained. The disadvantage of high Mg content is the decrease of surface tension which makes pores easy to bust. A good trade-off between the beneficial and adverse effects of Mg content for AlSi7 based alloy is 4wt%.