The work aims to study the factors affecting the cladding temperature variation of spent fuel assemblies during transportation to determine whether there are restrictions on the thermal load and loading position of high-burnup assemblies in spent fuel cask. The radial heat transfer of the spent fuel cask was simplified into the heat conduction of a cylinder with internal heat source to carry out theoretical analysis, and the CFX software was used to carry out three-dimensional temperature field analysis to further prove the conclusion of theoretical analysis. The difference in atmospheric environment temperature between hot condition and cold condition, the solar radiation and the decay of thermal load over time were the main reasons for the variation in cladding temperature. The difference in cask temperature gradient introduced by the deviation of thermal load between hot condition and cold condition was offset by the effects of the increase in thermal conductivity of the material selected for the cask and the spent fuel assembly with the increase in temperature and the enhancement of thermal radiation with the increase in temperature. The variation of the cask external surface temperature basically determined the variation of the cladding temperature, and the former was related to the combination of the emissivity and solar radiation absorption ratio of the cask external surface and might exceed 100 ℃. Therefore, it was necessary to analyze and demonstrate whether the cask loaded with high burnup assemblies met the criterion of cladding temperature variation. The emissivity of the external surface of the cask should be as high as possible, and the solar radiation absorption ratio should be as low as possible. Under an appropriate combination of the emissivity and solar radiation absorption ratio of the cask external surface, it can be ensured that the variation of cladding temperature under any thermal load and any loading configuration meets the criterion of cladding temperature variation, so there is no need to put forward requirements on the thermal load and loading configuration of loading high-burnup assemblies in the technical specification, which ensures the flexibility of loading.