Abstract: The deformation behavior and microstructure evolution of AZ31 magnesium alloy at the dynamic impact were studied by Hopkinson pressure bar testing,the spacing ring limit technology and microscopic observation,etc. At last,the plastic deformation mechanisms of AZ31 magnesium alloy were analyzed. The results show that the flow stress of AZ31 magnesium alloy increased with the increase of strain gradually in the range of strain rate from 1 524 s-1 to 2 024 s-1 and demonstrated the strain strengthening effect. The plastic deformation mechanisms of AZ31 magnesium alloy were transformed in turn by the slip and twinning,grain refinement,deformation localization with the increase of plastic deformation. Meanwhile,the temperature within shear band was about 241 K and met the twinning dynamic recrystallization′s temperature. The slip and twin rotation in grain are the main mechanism of grain refinement within shear bands,and the transient temperature rise promotes the grain refining.