Abstract: Sky background radiation is a critical parameter that constrains the sensitivity of infrared astronomical observations and serves as a key indicator for evaluating infrared observation sites. In order to meet the need of site survey for large aperture infrared telescope, a design scheme for a portable sky radiometer in the 8 μm~12 μm wavelength band was presented. The performance parameters of the optical system were established based on the requirements for measuring the temporal and spatial variations of 0.1% sky radiation. The optical system, which consisted of two lenses, was designed to minimize the thermal load on the Dewar cooling system. By using binary planes and aspherical surfaces, chromatic aberration and thermal defocusing were eliminated in operating temperature range. This ensured that the optical system achieved diffraction-limited （Airy spot radius was 30 μm） performance across various environmental conditions. The impact of optical element deformation and temperature gradients on aberrations under varying ambient temperatures was investigated through the integration of finite element analysis and optical simulation techniques. The results indicate that the aberration reaches its maximum at an ambient temperature of −60 ℃. The geometric radius of the spot diagram exceeds 42 μm, while the root-mean-square radius remains below 20 μm. The modulation transfer function at cut-off frequency is greater than 0.069 54, approaching the diffraction limit. Therefore, even without a focusing mechanism and window temperature control, the sky radiometer can satisfy the requirements for measuring the temporal and spatial variations of sky radiation.