in order to effectively improve the target detection and recognition ability of ir imagers, based on a 320x256 cooled staring focal plane array(fpa) detector, pixel size 30 mu mx30 mu m, a mid-wavelength infrared dual field of view zoom system was designed. in this paper, the working wavelength is 3 mu m similar to 5 mu m, the temperature range is -40 degrees c similar to+50 degrees c, this system can realize 200mm and 400mm dual focal length, the f-number is 2, the full field of view of short focal length is 3.44 degrees and long focal length is 1.72 degrees respectively, satisfy 100% cold shield efficiency. a re-imaging refractive system was adopted in this designed optical system consists of main optics and projection components. first of all, the structural selection and the initial parameter calculation were introduced in detail. secondly, on the basis of variety of the distance and temperature, a focusing len was presented in this system to adjust to produce a clear image. last but not the least, to improve image quality and environment adaptability, the analysis of temperature change and narcissus effect were described particularly. the design results prove that at the spatial frequency of 17 lp/mm, the mtf of the optical system is greater than 0.5(the axis mtf of the optical is greater than 0.6), the system can offer a high resolution and excellent images in whole range of the focal length, and it has the advantages of good adaptability, compact structure, high optical transmission and small size.