A method of quantitatively measuring fragment velocity based on a terahertz wave Doppler interference velocimetry (TDV) system is proposed in this paper. The theoretical model of fragment group velocity measurement is established, and the quantitative extraction method of fragment group initial velocity, velocity attenuation coefficient, and fragment velocity or distance change with time are given. Using this model, the extracted maximum and minimum values of the fragment group's initial velocity and attenuation coefficient are in good agreement with the preset simulated values. An experimental demonstration test shot of a cylindrical explosive device was carried out by using a TDV system, and the extracted initial velocity, velocity attenuation coefficient, and other important parameters of this explosive device fragment group are given in this paper.