Using Gleeble-3500 thermal simulator, the high temperature plastic deformation behavior and microstructure evolution of Mg-9Al-3Si-0.375Sr-0.78Y alloy are investigated at the temperature of 523 K?673 K and the strain rate of 10?3 s?1?10 s?1. True strain-true stress curves show the characteristics of the typical dynamically recrystallization process. The Arrhenius constitutive equation of the hyperbolic model is established. The average activation energy and the strain rate sensitivity index are,respectively,221.578 kJ·mol?1 and 0.137. The result shows that the α-Mg phase exhibits dynamic recrystallization(DRX)characteristics obviously. But no DRX occurs in the β-Mg17Al12 phase. Hot deformation does not affect the primary Mg2Si phase. Under the conditions of low temperature (523 K?673 K)and high strain rate(1 s?1?10 s?1),the flow instability and macro-defects such as crack appear in the specimens. However,there are finer recrystallization grains. Under the conditions of high temperature(≥673 K)or low strain rate,the microstructure of the alloy shows good homogeneity. The size of the primary Mg2Si phase is uniform,the size of theβ-Mg17Al12 phase is small,and the distribution of theβ-Mg17Al12 phase is uniform.