Abstract: On the basics of a cool 320240 detector with staring focal plane array, a large aperture off-axis catadioptric middle infrared continuous zoom system was designed. The system worked at 3.7-4.8 m which achieved the zoom of 250 mm to 2 000 mm and F number of 4. The optical system was divided into two segments. Firstly, an off-axis aspheric non-power system as the front system was designed. Then the continuous zoom lens which matched with the non-power system as the back system was designed. Because of the material constraint, the refractive zoom system could not realize large aperture. Coaxal catadioptric zoom system had great block ratio. And three mirrors zoom system could not achieve cold shield efficiency 100%. The off-axis catadioptric zoom system cold account for forenamed limitation. The system cold offer a high resolution and excellent images, and its cold shield efficiency is 100%. The system satisfies the design requirements.