Through the empirical electron theory of solids and molecules (EET), the valence electron structure of Mg-Al alloy melt was analyzed. Based on the analysis, the basic structure model was established and the melt structural information was obtained. By analyzing the irreversible phenomenon of viscosity-temperature relationship curve V(t) of magnesium alloy melt in the temperature interval of ascent and descent, the irreversible change of the melt structural information would be judged indirectly. The results show that the number difference of covalent electron pair nα of each bond between atoms is the ultimate reason of the different melt structural information, which induces micro-inhomogeneity in alloy melt directly; at near 730 °C, Al-MgⅢ(C) bonds in the atomic clusters with relatively stable structure begin to break. At this time, the melt structure achieves a new equilibratory and relatively homogeneous state. Disappearance of micro-nonequilibrium structure inherited from solidified structure, breakage of Al-MgⅢ(C) bonds in the atomic clusters and irreversible rearrangement of atoms are the essential reason of irreversible change of curve V(t). It is very important for further understanding the melt structural information of magnesium alloy and exploring solidification micro-mechanics.