Using the technology of split Hopkinson bar and a new method that can be used to interrupt test and obtain different amount of strain, dynamic compression test has been done on TC6 titanium alloy. By associating microstructure of shear band under different amount of strain with mechanical response of material, the formation cause of stress collapse in TC6 alloy under dynamic compression has been studied. It is shown that formation of adiabatic shear band of materials under high strain rate is a process composed of initiation, expansion and full development, and that stress collapse in stress-strain curvy is not resulted from the initiation and growing of adiabatic shear band, even involving cracks with its size not larger than certain critical size. Real reason of stress collapse in curvy is due to size of cracks has exceeded certain size within the adiabatic shear band.