In order to avoid the influence of the interference noise introduced by the front and rear surfaces while measuring the parallel plate, an absolute measurement method for the optical homogeneity was proposed based on the principle of phase shifting by lateral displacement of the point source. Three steps of the proposed method are as follows: the front and rear surfaces of the parallel plate interference measurement, the transmission wavefront through the parallel plate measurement and the empty cavity of the interferometer measurement. The wavefront is recovered by four phase-shifting interferograms captured at the same time and the homogeneity distribution of the parallel plate could be calculated according to the measurement results. An optical parallel plate with a thickness of 60mm was tested on a vibration non-isolated platform and the measurement result indicates that the peak-to-valley value is ΔnPV=3.32×10－6 and root-mean-square value is ΔnRMS=2.63×10－7. Compared with the results obtained by the wavelength tuning interferometer, the deviation of the peak-to-valley value is ΔPV=5×10－7 and the deviation of the root-mean-square value is ΔRMS=－7×10－9. The measurement results are in good agreement with those obtained by wavelength tuning interferometer. The measurement accuracy of homogeneity obtained by the proposed method is 1×10－6 under vibration conditions.