Based on a method named Orthogonal Polarization Interferometry,testing system on refractive index nonuniformity of KDP crystal is proposed,and phase detuning angle can also be indirectly tested.Polarizaton state of the testing light is precisely controlled and switched in the interferometer.Error introduced by tilt of the optical element is avoided by adopting wavelength-tuning phase shifting method.Phase shifting algorithm is optimized to raise testing precision and repeatbility.Analytic algorithm of index of refraction nonuniformity is designed,and therefore the error introduced by variation in the crystal thickness is avoided.The measurement result for the crystal with small aperture shows that the accuracy of the instrument is better than 10-6 (RMS).