A new synthetic heterodyne laser Doppler interferometer based on homodyne interferometer is addressed in this work. Interference signal includes the DC offset that is light intensity. The DC offset must be got rid because it does not need to obtain a velocity. The high pass filter is used instead of the subtractor. DC offset having random variation is difficult to get rid as using the subtractor itself. When the vibration amplitude is smaller than at least 1/ 2 of the wavelength of He-Ne laser, a serious problem of incorrect velocity measurement can be caused since there is non-zero crossing interference signal whose DC value can be eliminated in using the HPF. In order to solve this problem of using the HPF in the homodyne interferometer, a synthetic interferometer using a mechanical modulation method is proposed in this work by exciting a reference mirror with the displacement larger than the 1/2 of the wavelength. In this work, the analytical work is presented to show how the synthetic interferometer solves the problem of incorrect velocity measurement using the Fourier-Bessel function description of the interference signals. Simulation and Experimental works are also presented to validate the synthetic heterodyne interferometer proposed in the work.