For 320 pixel×240 pixel cooled staring focal plane array detector, a mid-infrared zoom optical system with ratio of 25 is designed. The system consists of a zoom system and a re-imaging system. The zoom system consists of two small zoom lenses, which can realize large zoom ratio; the re-imaging system can satisfy requirement of the cold shield efficiency, and the diameter of lens can be reduced because of it. The mid-infrared zoom optical system works in the range of 3.7～4.8 μm, it can realize 12～300 mm continuous zoom and satisfy requirement of 100% cold shield efficiency; F number of the system is 2.5. The zoom system has advantages such as a large zoom ratio, a large aperture, and short and smooth zoom paths.