Abstract: Three-field zoom system has advantages of simple mechanical structure, better reliability, short time of changing focal relative to other pattern zoom systems. Using 320240 resolution cooled detector with 30 m30 m pixel dimension, a middle-wave three-field optical system was designed by secondary imaging way. The system's F number is 4, FOV range is 1.4-23.8, it can realize 30 mm/100 mm/500 mm three position focal length. In designing process, the Germanium material and Silicon material were adopted to balance chromatic aberration, introducing one asphere to balance sphere aberration. The system use two mirrors reducing the axis dimension. The dimension of system better than 210 mm160 mm120 mm, it has characteristics of small dimension, simple zoom structure, good image quality etc. The modulation transfer function(MTF) is above 0.5 at spatial frequency of 17 lp/mm and energy concentration ratio is greater than 70% within the sensing element of the detector.