The differential technology of Laser Doppler and the frequency-tracking technology of PLL are studied and a Laser Doppler differential demodulation system with high accuracyand broad dynamic range is designed. The system has the characters of tight structure, convenience on fixing and tuning and can make full use of laser energy. Furthermore, by focusing the waists of two Gauss beams on the object measured, the smallest speckle and the superposition of plane waves are obtained to enhance the resolving power, SNR and the intensity of signal. It tracks the frequency automatically to resolve the dropout of signal and to restrain all kinds of noises and disturbances. It is proved by simulation experiment that the measure accuracy of amplitude and frequency respectively are 1% and 0.6% and the resolving power is 50nm, so the technology indexes are satisfactory.