The refractive index of infrared material is measured using the infrared Twyman-Green interferometer.By adding a rotation stage in the test arm,the OPD of the test piece placed on the stage is changed with the rotation and the interference fringe is shifted.The refractive index of the test piece can be calculated by the recorded values of rotating angle and shifting fringe.The result shows that the refractive index of germanium crystal is 4.003 and the refractive index of GeAsSe is 2.494 when the temperature is 25 ℃ and the wavelength is 10.6 μm.The measurement error of the method is 10－3 ,which indicates that the precision can be improved if the test piece is thicker or its refractive index is lower.