The standard "GB/T 8758-2006 GaAs epitaxial film thickness infrared interference measurement method" defines an infrared interference technique for the measurement of GaAs epitaxial film thickness, which is suitable for epitaxial layers with film thickness greater than 2 microns. The method relies on infrared light reflection on the surface of the epitaxial wafer to generate interference fringes, and calculates the epitaxial film thickness by analyzing the wavelength position of the interference fringes and Optical inspection constants. The standard requires the use of a dual beam of light Infrared spectrophotometer or Fourier Infrared Spectrophotometer with a wavelength range of 2.5 microns to 50 microns, and the wavelength error of the instrument does not exceed 0.05 microns. Optical inspection surfaces and no large passivating layers are required. Detailed steps for instrument calibrating and measurement processing are also specified.