A 4× continuous zoom optical system with high resolution is designed for the application of the long wave uncooled focal plane array (FPA) detector with 1 024×768 pixels. The zoom process of the system is analyzed by using a macro program written by ZEMAX macro language. The mechanical compensation zoom mode is used in the system, and there are six pieces of germanium lenses with three aspheric and one diffractive surface. The high order aberrations is well corrected and the system can zoom smoothly and continuously in the focus length range of 25~100 mm with the viewing angle from 32°×24.3° to 8.2°×6.15°. From the zoom cam curve output from macro program, diffuse spot curve and field distortion curve, it can be seen that the system runs smoothly in the whole zoom process with a high-quality stable image, the various of the diffuse spot and the field distortion is small. In conclusion, the system has the advantages of compact structure, large relative aperture (F#1~1.4), small field distortion, high image quality, and easy to manufacture the zoom cam mechanism, and has high application value.