Abstract: Aiming at the cooled 320256 detector with staring focal plane array (FPA), an optical system of middle infrared continuous zoom system was designed for airborne optoelectronic detection and tracking apparatus. The system was composed of a zoom lens system, a secondary imaging system and two reflectors. In order to avoid the fabrication and testing error of aspheric surface, the whole system was made of nine spherical glasses. The design results indicate that the system achieves the continuous zoom of 37.5 mm~150 mm, with working length of 3.7 m~4.8 m, and F number of 4, which can obtain the cold shield efficiency of 100%. At the Nyquist frequency of 16 lp/mm, the MTF values of all focal length positions and field of view are more than 0l55, which are closing to the diffraction limit. The overall lens length is 280 mm and the weight is only 110 g. The optical system has the advantages of small volume, high image quality and simple structure.