To solve the multiple-surface interference fringe pattern admixture problem in measuring parallel plate with phase-shifting interfermetry (PSI), the basic principle of multiple-surface interference fringe analysis method with wavelength-tuning phase-shifting is introduced. As the three surfaces interference for example, the front and back surfaces can be got by analyzing the interferograms by Fourier transform and getting the phase of the corresponding spectrum. The maximum error and variance distributions with different sampling buckets and light spectral ranges are compared. In ideal case, the error of this method is lower than 10-4λ. Experiments on an optical element by this method and Zygo′s interferometer, the result shows that the relative errors of surface peak-to-valley (PV) and root mean square (RMS) is lower than 1%.