In order to obtain the two dimensional distribution of the refractive index changes due to the domain inversion in the RuO2:LiNbO3 crystal, we make use of the Mach-Zehnder interferometer and 4f optical lens system to record holograms, with the object light transmitting through the poled RuO2:LiNbO3 crystal, and then filter the spectrum in the frequency domain to get the object light spectrum for numerically reconstructing the object wavefront. The results confirm that the electro-chromic effect region well coincides with the domain inversion region in RuO2:LiNbO3 crystal. Digital holographic interferometry has the merits of being nondestructive and it provides a prospective way to in-situ visualization, monitoring and analysis of the domain inversion in LiNbO3 crystal.