The method is presented to measure the homogeneity of the refractive index of infrared optical material, germanium crystal, using infrared interferometer with 10.6 μm wavelength laser as light source. Using traditional interferometric method, as the germanium material is too soft to process, the surface form of the test piece may not meet the requirement for the measurement. The digital phase-shifting interferometer with visible light source is an accurate instrument of which uncertainty is within λ/50 (λ=0.633 μm). The surface distributions of test piece are accurately obtained using the phase-shifting interferometer with visual light source and surface errors are subtracted during the data processing to reduce the affection from the surface form deviation during the interference test. A piece of germanium crystal is tested and the deviation distribution of the refractive index is presented.