Abstract: Based on the test information from multi-sample and multi-mode instrument, as well as the guide to expression of uncertainty in measurement (GUM) and Monte Carlo method (MCM) , the four-step interferometry measurement range and uncertainty of the infrared optic materials refractive index uniformity were discussed. Although the absolute sensitivity and accuracy of infrared interferometry are lower than that of visible laser interferometer, the four-step interferometry can eliminate the inherent systematic errors of the interferometer and can help achieve high-accuracy measurements of infrared optical refractive index homogeneity. The practical tests show that the measurement range can reach 110-5～510-3 and the relative standard uncertainty can reach 210-1～210-2.