To resolve the contradiction between the measurement of micro-displace or high-accuracy detecting of micro-vibration of ultra-low frequency and the convenience of building and adjusting, a inverted differential 4f grating demodulation system of laser Doppler is put forward. Using two phase gratings to realize frequency shift and mixing, the measurement accuracy is enhanced. Adopting 4f configuration to enhance the spatial coherence, building and adjusting of the system are more convenient. Further more, adding λ/4 wave plate and Wollaston lens cooperating with differential amplifier in order to get sin, cos signals judging displacement direction, and that the energy of both ±1 and ±3 diffraction lights is taken use to improve the intensity and signal to noise ratio (SNR) of Doppler signal.