High ratio zoom lens design using three-group flexible structure is researched. The Guassian optics for three-group structure with fixed middle group is discussed. The range of Guassian parameters and principle of selection for high ratio design are presented. The visible assist design software for Guassian parameters selection is developed. A flexible zoom lens with ratio 50×, focal length 12~600 mm, and field of view 0.85°~41.2° is designed. The optical total length is 175 mm for short focal length, 309 mm for long focal length, and the telephoto ratio of long focal length is 0.51. The modulation transfer function (MTF) at 100 lp/mm for each focal length is higher than 0.3 at center field, and higher than 0.2 at edge field. The system possesses the characters of miniature and portable.