On the basics of a cool 320×240 detector with staring focal plane array, a offaxis catadioptric middle infrared continuous zoom system with large aperture and long focal length is designed. The optical system was divided into two segments. First 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 can′t realize large aperture. Coaxal catadioptric zoom system has great block ratio. And three mirrors zoom system can′t achieve cold shield efficiency 100%. The offaxis catadioptric zoom system can account for forenamed limitation. The system worked at 3.7～4.8 μm has achieved the zoom of 250 mm to 2 000 mm and F number of 4. This system can offer a high resolution and excellent images, and its cold shield efficiency is 100%. The system satisfies the design requirements.