The transmission curves of HgCdTe wafers with different thicknesses were measured by FTIR. Its composition and cut off wavelength at 80K were determined by using the empirical formula. The actual cut off wavelengths were measured and obtained from their spectral responses. The result shows that the relative deviation between predicted cut off wavelength and actual one is 2.5%. Therefore, this method can provide one of the most important parameters for material selection of HgCdTe for device manufacture.