An off-axis three-mirror collimating optical system is designed to test the infrared equipment in a vacuum 100-K cryogenic environment. The system adopts athermal design， and the whole structure adopts SiC material. A C-shaped opening expansion sleeve is used as a flexible structure to compensate for low-temperature deformation at the connection between the mirror body and the support structure. The whole system is partially wrapped in a radiation cooling panel， except for the reflecting mirror. The low-thermal conductivity thermal insulation support structure plays a thermal shielding role in the heat conduction chain to realize the thermal insulation support and rapid refrigeration of the system. In the 100-K low-temperature environment， according to a system simulation analysis， the primary， secondary， and tertiary mirror wavefront error is <λ/50； an analysis of the whole structure indicates that the wavefront errors of the primary， secondary， and tertiary mirrors are ≤λ/30. The wavefront error of each field of view of the system is in the range of λ/14-λ/8 at room temperature and in the range of λ/8-λ/7 at 100 K. According to the Rayleigh criterion， the wavefront is considered to be flawless. When λ = 632.8 nm， the modulation transfer function （MTF） of the system at a 50 lp/mm frequency is >0.7 at room temperature； at a low temperature， the MTF is >0.6. Meet the system at 50 lp/mm greater than 0.6 use requirements. The results indicate that the collimating optical system can output parallel light stably in a 100-K vacuum environment to meet the test requirements of low-temperature infrared equipment. In a material level test of fast radiation cooling， the temperature of the SiC mirror blank is stable at 130 K after 18 h. After 30 h， the mirror blank temperature stabilizes at 110 K. The experiment verifies the feasibility of fast radiation cooling of the SiC mirror.