Abstract: The thermal conductivity of nuclear fuel cladding is an important parameter for the study of heat transfer performance of nuclear fuel elements and nuclear safety design. The double-layer structure of SiC composite cladding and the properties of SiC/SiC composite material on its inner layer increase the complexity of predicting the thermal conductivity of the cladding. To predict the thermal conductivity of the SiC double-layer composite cladding and inner-layer composites, based on the Markworth effective-medium theory, the effects of porosity, temperature, neutron irradiation and other factors on the thermal conductivity of SiC/SiC composites were considered, the thermal conductivity of different types of SiC/SiC composites made of second and third generation SiC fibers was calculated on the basis of the modified Markworth effective medium model. The thermal conductivity calculation model of the SiC double-layer composite cladding was established, and the thermal conductivity of the SiC double-layer composite cladding during the steady-state operation of nuclear power plants was initially predicted. The calculation results show that after 4 months of steady-state operation of the nuclear power plant, the irradiated swelling deformation reaches saturation, the thermal conductivity of the cladding containing the second generation fiber is 3.9 W/（m·K）, and that of the third generation fiber is 4.8 W/（m·K）.