Abstract: Structure change rules of static and dynamic recrystallization on thermal deformation of AZ31B magnesium alloy thin sheet were investigated by thermal stretch machine,metalloscope and SEM.The results show the static recrystallization before deformation and the dynamic recrystallization in the deformation are observed in the thermal stretch of magnesium alloy.The time of static recrystallization before deformation is 12-18 min,at the deformation temperatures of 240-300 ℃,and the times of thermal stretch were 10-25 min,at the strain rates of 1.000×10-3s-1,evenly distributed and refined equiaxed grains could be obtained under these conditions.Fractures are composed of tearing ridges,voids and alone refine grains.