Abstract: For middle infrared cooled 640512 detector with staring plane array, a middle infrared dual field of view optical system with long focal length and high resolution was designed. The wavelength range was 3.7-4.8 m, F number was 4.0, long focal length and short focal length was 800 mm, 400 mm respectively. Reimaging technology had not only minished the diameter of front fixed group, but also met 100% cold shield efficiency. Two infrared materials Ge and Si were used in the optical system and two aspheric surfaces were used to correct the off axis aberration and higher order aberration. Moreever,the Modulation Transfer Function(MTF) was above 0.2 both the long focal length and short focal length at the spatial frequency 33 lp/mm. The distortion is below 0.5%. The results show that the middle infrared dual field of view optical system has small volume, high resolution and excellent image.