The design of an infrared zoom optical system based on 480×6 focal plane array detector was introduced. The zoom theory, diffractive theory and aspheric design method were used in the aberration balance. The operating wavelength region was 7.7 to 10.3 μm, F-number was 2.62, and zoom ratio was 15 to 1. The result indicates that the zoom optical system could obtain better optical performance in all zoom range, by the general analysis and evaluation of modulation transfer function, distortion curve, diffractive optical element, and zoom curve analysis. The overall length of optical system is 480 millimeter, back focal length is 28 millimeter, modulation transfer function is more than 0.3 and distortion is less than 5 percent.