Abstract: Infrared optical system is the core component of infrared imaging devices. Athermalization is widely adopted in mid-infrared optical design in order to improve performance and stability of infrared imaging optical systems, and meet performance requirements of civil and military applications. In this paper, passive athermalization design of a non-thermal mid-infrared imaging optical system is realized through combining different infrared materials. Contribution of narcissus is calculated by NITD method, curvature and optical power of surface with large contribution to narcissus are optimized. Results show that MTF of optical system is greater than 0.5 at 30 lp/mm in temperature range of -40 ℃ ~ 60 ℃, defocus distance is less than depth of light, RMS diameter is less than pixel size. Maximum NITD value of narcissus is reduced by 40%, which is smaller than minimum resolution temperature difference of the system. Application of this method can be very good to achieve athermalization design of infrared imaging optics system, and suppression effect of narcissus is obvious.