Abstract: Compacted bentonite is an ideal buffer/backfill material for high-level waste repositories, and its hydration swelling pressure is critical to structural stability of the repository. Compacted bentonite has multi-scale structural characteristics and exhibits flocculated structural features within the aggregate, where the swelling pressure generated during its hydration is a result of the combined effect of the montmorillonite swelling and the flocculated structural adjustment. This work is devoted to a cross-scale theoretical model of the hydration swelling pressure in bentonite aggregates. The structure of aggregates is divided into flocculation units with their equivalent physical-mechanical properties calculated; The probability distribution of each flocculation unit in aggregate structures is solved based on the principle of minimum energy and maximum entropy; By analyzing the mechanical connection between the layers and the flocculation units, and between the flocculation units and the flocculated structure of aggregate, the cross-scale connection between the layers and the flocculated structure of aggregate in terms of physical-mechanical properties is realized. The theoretical prediction model for the hydration swelling pressure of aggregates is established. At the same time, the fine-scale physical-mechanical 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 show that the model applicability is well. 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.