Phase equilibrium of the Mg-Al-Sn alloy system was calculated in detail by utilizing phase diagram calculation commercial package and other thermodynamic data for Mg alloys. The calculated phase diagrams were compared with the DSC results for the alloys of Mg-3wt%Al-3wt%Sn. The solidification and precipitation process of Mg-3wt%Al-Xwt%Sn were analyzed. The detailed comparison strongly supports the reliability of the selected thermodynamic description based on the results of thermodynamic calculation. It is clarified that when the Mg-Al-Sn alloy ends its solidification and cools down continuously, the phase Mg2Sn precipitated first from solid solution of magnesium, and then the Mg17Al12 phase precipitated at relative lower temperature. This is shown that the calculation phase diagram method considerably reduces the effort of alloy design and that the reliability of the results is high.