Abstract: A refractive infrared zoom optical system working in (8～12)m is designed, based on long-wave uncooled thermal IR focal plane arrays (160 pixel120 pixel) detector. The system has characteristics of large relative aperture, F#=1.2, high zoom ratio and high imaging quality. Two common infrared materials of Ge and KCl are used in this optical system. The off axis aberration and higher order aberration of the system are corrected by introducing an aspheric surface into the system, and the zoom ratio is rapidly improved by solving the compensated curves at the middle-focus length. The modulation transfer function (MTF) is above 0.55 within the whole focal range at the spatial frequency of 17lp/mm and approaches the diffraction limit. The energy concentration ratio is greater than 72% within the sensing element of the detector whose receiving radius is 17.5m. The above mentioned parameters show that the optical system has good imaging quality.