Abstract: In order to deeply analyze the void and crack in adiabatic shear bands(ASB) of Mg alloy at high strain rate, the formation mechanism of micro cracks in adiabatic shear bands was studied. The split hopkinson pressure bar (SHPB) was used to carry out high speed impact experiment for the hat specimen of AZ31 magnesium alloy. The microstructure of the samples after impact was observed by OM, SEM and TEM. The process of the formation of micro cracks in adiabatic shear bands were discussed.The results show that the adiabatic shear band is formed at the maximum shear stress area.The void is firstly formed in ASB.With the increase of the impact loading, the void extend.