An on-axis,vibration insensitive,polarization Fizeau interferential system is built. In this arrangement,a multimode semiconductor laser is used as the light source,of which coherence in conjunction with a polarization delay-line is used to effectively separate out the orthogonally polarized test and reference beam components for interference. Spatial phase-shifting system is realized by a 2D grating and a polarizer group. The four ties of rays including two ±1 diffraction rays with the same intensity are projected through four polarizers,of which the axis directions are 0°,45°,90°,135°,and four π/4 phase shifting interferograms are received by one CCD simultaneously. By using the 4-bucket algorithm,wavefront to be tested is recovered. Flatness of a parallel plate surface is measured by this system with PV of 0.068 λ,RMS of 0.013 λ (λ=650 nm) and by Zygo interferometer with PV of 0.063 λ,RMS of 0.012 λ(λ =632.8 nm). The two results obtained are in close agreement.