Due to the vast territory of China, the environmental temperatures in different regions vary greatly. In order to meet the environmental adaptability requirements of the infrared thermal imager for safety monitoring of building engineering, the passive uncooled design mode of refraction and diffraction mixed optics is adopted. Through the reasonable distribution of optical focal degree of the lens with different materials, a miniaturized uncooled infrared imaging optical system is designed, which can work in the range of -55 ~ 70 ℃ and has a field-of-view of 24.1°×19.4°. The operating wavelength range is 8~12 μm, and F number is 1.0.The design results show that the system has simple structure, small volume and good imaging quality. The value of optical modulation transfer function(MTF)at the spatial frequency 42 1p/mm is greater than 0.3, which can meet the application requirements.