The long-wave infrared detector is the key sensor of the airborne Infrared Search and Tracking (IRST) system, and its imaging performance directly determines the key parameters of IRST, including the target detection, and finally affects the infrared detecting distance of IRST. A long-wave standard imaging lens is designed to evaluate the imaging performance of the long-wave infrared detector with a focal length of 150 mm and a total length of 210 mm. The distortion rate of it is less than 2%, and the Modulation Transfer Function (MTF) is up to the diffraction limits. The imaging lens can accurately and quantitatively evaluate the imaging quality of the infrared detector during the screening and checking process.