Abstract: The refraction index inhomogeneity of KDP crystal can cause beam phase mismatch in spatial distribution, thus lead to decreasing conversion efficiency of the third-harmonic generation system. In order to obtain the refraction index inhomogeneity of KDP crystal in high precision, the orthogonal polarized interferometry (OPI) was adopted. Using the ZYGO large aperture interferometer, the refraction index inhomogeneity distribution of large size KDP crystal was attained, with the precision as high as 10-7, the effect of crystal surface profile on the testing result was also studied experimentally. The high precision measurement of the refraction index inhomogeneity of crystal provides quantitative measurement references for the improvement of growth and machining technique of large aperture crystal materials.