A Cassegrain optical system of infrared search and trace (IRST) working at 8-12 μm is introduced.After analyzing the characters of the IRST system,the compositions of the optical system are described.A diffractive optical element (DOE) is successfully introduced to the second imaging to correct ray aberrations,which makes the system working well at room temperature.In order to make the system work in a wide thermal environment,an electromechanical device is utilized to control the compensation lenses.To suppress the great amount of stray light,lens hood is set on the primary and secondary mirrors,which could eliminate stray light efficiently.Hence signal-to-noise ratio and contrast of the picture have been enhanced.The experiment shows that the system works well.