The basic principle and experimental method of Orthogonal Polarization Interferometry (OPI) were described in this paper.The output laser beam of Fizeau interferometer was polarized by a linear polarizer,and two interferograms were obtained:the polarization orientation of one of them consisting with o-axis of KDP crystals,and the other one polarizing orientation is orthogonal with that o-axis.The non-uniformities of refractive indices of KDP crystals were calculated from difference of those two interferograms.The large aperture interferometer was used to accurately measure the differential distribution of KDP refractive indices.A linear polarizer whose polarizing orientation was changeable was put at the end of small aperture of interferometer.The difference of KDP refractive indices of eight pieces of KDP crystals as large as 330mm×330mm was obtained by orthogonal polarization interferometry.