On the basis of a cool 320×240 detector with staring focal plane array, a set of middle infrared continuous zoom system is designed for airborne optoelectronic detection and tracking apparatus. The system is composed of a zoom system, a secondary imaging system, and two reflectors. The method of pupil-join for a secondary imaging system is introduced and the schematic diagram and its parameters are given by using an optical design software, then the image quality and Narcissus effect are analyzed. The design results prove that the system worked at 3-5 μm has achieved the zoom of 10 mm to 180 mm, large zoom ratio of 18× and the F number of 3. The system can offer a high resolution, heat sensibility and excellent images, and its cold shield efficiency is 100%. The compact system meets the design requirements.