Using the technology of split Hopkinson Bar, the dynamic shearing experimentation on Ti-5Al-2.5Sn alloy has been conducted. The specimens are hat-shaped. Formation mechanism of adiabatic shear band of Ti-5Al-2.5Sn alloy has been studied by optical microscope and transmission electron microscopy (TEM). Results show that twins play a key role in the formation process of adiabatic shear band of Ti-5Al-2.5Sn alloy. The formation process of the adiabatic shear band can be divided into three stages. In the first stage, the plastic deformation of the forced shear zone is accomplished by dislocation slip and twining shear under impacting loading. A great deal of twins are formed in the deformed area. In the second stage, the local crystal orientation is adjusted due to the formation of deformed twins so that the dislocation that is at a disadvantage orientation of slipping formerly starts up, resulting in formation of the elongated sub-grains in the shear area. In the last stage, owing to interacting of the mobile dislocation and the twins, the fine equiaxial grains are formed by fracture of the twins under dynamic loading. At the same time, the fine equiaxial orthoscopic dynamic-recrystallized grains with fewer dislocations are formed in local areas of the shear band center. Size of the fine equiaxial grains formed in the shear band of Ti-5Al-2.5Sn alloy is approximately 0.1-0.3 μm