A Fabry-Perot (FP) resonant technique is proposed to measure the quadratic electro-optic coefficients γ33 and γ13 of transparent lead magnesium niobate titanate (PMNT) ceramic for transverse electric (TE) and transverse magnetic (TM) modes. The resonant wavelength in the PMNT FP resonant cavity will shift with the increase of applied voltage due to the quadratic electro-optic effect. The quadratic electro-optic coefficients γ33 and γ13 for TE and TM modes are measured, and their values are about 21.06×10-16 m2/V2 and -1.76×10-16m2/V2, respectively. Using the characteristic, a polarization-dependent PMNT electro-optic diffraction grating is designed and the diffraction patterns for different polarization light are also analyzed and compared in detail.