This paper reports a design process for high magnification mid-wave infrared continuous zoom lens. The process consists of algorithms for finding global paraxial solution, optimization strategies for balancing aberrations in all zoom configurations, and cam curve smoothing methods in opto-mechanical design. Using this process, a parfocal continuous zoom lens with F/4, magnification of 20× working with a 640x512 cooled 15μm pixel pitch detector array, has been successfully designed and fabricated. In principal, this design bases on a zoom and a focus group. The zoom group consisting of three separate moving lens subsets allows for a high magnification ratio of 20× and its parfocality during continuous zooming, while the focus group helps changing focal planes and ensures normal operation in a large temperature and observation range. The lens uses different optical surfaces varying from spherical to hybrid diffractiveaspheric form. This results in high optical performance with MTF approaching diffraction limit while keeping the overall system compact and light weight. For the coordination of lens movement in the zoom group, smooth cam curves having satisfactory stiffness and pressure angles were achieved by a smoothing method which maps different system focal length values with appropriate barrel rotation angles. The simulation of optical performance and experimental testing results will be presented. This zoom lens can be used in long range surveillance camera system.