In order to solve the problems in high zoom ratio lens design, a two-stage-of-series zoom lens model was proposed based on optics zoom theory. Therefore, differential zoom equations were deduced and solved to help the lens power assignments, also the cam curve optimization condition and method were proved. In this model, first stage zoom was achieved by the two-part-zoom front part, and the given movement pattern of the compensation lens and the second magnified lens determined the second stage zoom, by which the 100% cooled stop efficiency was assured. Based on this method, a 70x MWIR zoom lens under the requirements of F/4, focal length 6.5-455 mm, HFOV 0.92°-58.2°, work waveband 3.7-4.8 μm was designed, which only used two kinds of infrared materials, consisted of 10 lenses, total length 300 mm, had good image quality and tolerance character.