Abstract: Carbon fiber reinforced composites are ideal materials for spacecraft cryogenic equipment support structures because of both excellent mechanical properties and low thermal conductivity. However, the thermal conductivity of composites has significant temperature dependence, which affects the cryogenic safety analysis of composite structures. Meanwhile, the composite with complicated internal microstructural features lacks an effective thermal conductivity prediction method, which is not conducive to the adiabatic design of the composites. For this reason, this paper carries out the research on the experimental characterization and theoretical prediction method of cryogenic thermal conductivity of composites. The cryogenic evolution of thermal conductivities of composites and epoxy resins were investigated based on the experimental characterization, and the influencing factors of thermal conductivity of composites were revealed. By introducing the fiber crowding degree, a transverse thermal conductivity theoretical model considering the effect of fiber random distribution was constructed, which effectively improved the prediction accuracy of the transverse thermal conductivity of composite. The results showed that the longitudinal and transverse coefficients of thermal conductivity of the unidirectional laminates were reduced from about 6.32 Wm⁻¹K⁻¹ and 0.63 Wm⁻¹K⁻¹ to 0.36 Wm⁻¹K⁻¹ and 0.13 Wm⁻¹K⁻¹ when the temperature was reduced from 293 K to 20 K, respectively. The thermal conductivity of epoxy resin decreased from 0.32 Wm⁻¹K⁻¹ at 293 K to 0.14 Wm⁻¹K⁻¹ at 20 K, which is a smaller decrease than the composites. The cryogenic transverse thermal conductivity of T800 carbon fiber was also calculated by inversion of the theoretical model, which provides parametric support for the cryogenic thermal design of composite structures.