Abstract: A new transmittance measurement and assessment method of the infrared optical material at diffident temperatures was proposed. A collimation reflect mirror was used to make the infrared light source into the parallel light, and then a chopper was used to make the continuous light into the pulse light, besides, a special wavelength was obtained by filter. A special data acquisition system that including a preamplifier circuit, a lock-in amplifier, a computer and a control software could calculate the value of infrared transmittance and give the temperature coefficient of the optical material by means of double light beam contrast measurement. The method resolved problems of infrared transmittance measurement of optical material in actual operational environment, like high temperature. Test result shows the transmittance of sapphire material is higher than 10% at the wavelength of 519 5 nm, and the repeatability is better that 0.2% from room temperature up to 700℃. The method has great economic and social utility.