The optical system of single infrared spectrum receives less target information when the UAV reconnoitres the ground, and it is also susceptible to the influence of the environment or even unable to detect the target, which seriously affects the efficiency of reconnaissance. By analyzing the imaging principle and design method of the multi-spectral infrared optical system, the reflective structure is selected to design an off-axis three-mirror infrared optical system with a focal length of 50 mm and a full field of view of 12°×10°, which can be used for short-wave infrared imaging, medium-wave infrared imaging and long-wave infrared imaging. Meanwhile, the MTF value of the system is close to or reaches diffraction at the Nyquist frequency of 30 lp/mm in each spectral and each field of view, and its RMS value of the dispersion spot is less than the pixel size of the detector, and all indexes meet the requirements of imaging quality. This discovery expands the detection spectral range of the UAV system and improves the reconnaissance capability of the system.