Abstract: Compacted bentonite is an ideal buffer/backfill material for high-level waste repositories, and its hydration swelling pressure is critical to the structural stability of the repository. This work is devoted to a cross-scale theoretical model of the hydration swelling pressure in bentonite. Firstly, the structure of bentonite is divided into microstructure units with their equivalent physical-mechanical properties calculated; secondly, the probability distribution of each microstructure units in soil structures is solved based on the principle of minimum energy and maximum entropy; then, by analyzing the mechanical connection between the particles and the microstructure units, and the microstructure units and the structure of soil, the cross-scale connection between the aggregate particles and the structure of bentonite in terms of physical-mechanical properties is realized. At last, the theoretical prediction model for the hydration swelling pressure of soil is established. At the same time, the hydration swelling test was performed on Gaomiaozi bentonite to measure its hydration swelling pressure. The applicability and accuracy of this theoretical model were examined by comparing the measured values with the predicted values. The results showed that the model applicability was well, the error between the predicted value and the experimental result is less than 16 %. The theoretical model also provides a new idea for the research on the cross-scale structural properties of bentonite. It could be used for the prediction and evaluation of the hydration swelling characteristics of buffer materials, which would contribute to the design and construction of repositories.