An equivalent mass source is modeled as excited sources of internal waves generated by the displacement of fluid and the wake of vortex shedding or turbulence due to the body moving in a stratified fluid, a mathematical model for the vertical displacement of internal waves generated by such an equivalent mass source is presented by use of both the associated eigenvalue problem and the Fourier transform method, and the method for estimating the speed, length and diameter of the equivalent mass source is proposed. Calculations based on the proposed method are performed for the generation of internal waves by a towed sphere in a stratified fluid, and the numerical results are in good agreement with the available experimental results of Robey in the kinematic wave patterns and the maximum peak-to-peak amplitude of the internal waves. Such a method may have potentially important applications.