Abstract: Based on a 320240 middle-wave infrared cooled detector, a dual field of view infrared system was designed. Using focal shift of optical passive compensation temperature, athermalization design technology was realized. The computation process to obtain structural parameters of the optical system was introduced, and then optimization was applied by using Zemax software. The optical simulation and image evaluation were carried out. The experimental results indicate that the system realize the zoom of 50 mm and 200 mm at the band of 3.7m ~4.8 m by using eight lenses, and cold shield efficiency is 100%. During the temperature range of -20℃~60℃, value of the modulation transfer function is all above 0.5 at the Nyquist frequency.