Abstract: The phase transition temperature points of AZ81 magnesium alloy were measured by differential thermal analysis experiments, and the microstructure at phase transition points was obtained through liquid quenching method. The solidification behavior and microstructure evolution were analysed by optical microscopy, scanning electron microscopy and energy disperse spectroscopy. The results show that the solid-liquid phase transition temperature and eutectic transition temperature of AZ81 magnesium alloy were about 590 ℃ and 420 ℃, respectively. During the solidification of AZ81 magnesium alloy, α-Mg dendrites would precipitate firstly, aluminum content increased at the forefront of solid-liquid interface, eutectic transformation of Al-rich residual liquid occured at dendrite clearance, and finally solidified to eutectic microstructure of α-Mg and β-Mg17Al12. Moreover, as the eutectic microstructure was few, nucleation and growth of eutectic α-Mg phase were often dependent on α-Mg primary phase, and divorced eutectic microstructure formed through pushing β-Mg17Al12dendrite to the grain boundary of α-Mg.