Abstract: The compacted bentonite is widely used as the sealing barrier material, and its swelling pressure is regarded as an important design index. A device based on vapor equilibrium technique is designed for measuring the swelling pressure of expansive soils in unsaturated environment. The nuclear magnetic resonance (NMR) and X-ray diffraction (XRD) techniques are used to analyze the correlation mechanism between water distribution and macroscopic swelling behaviors of the bentonite during wetting process. The test results show that under the relative humidity control, with the decrease of the suction, the swelling pressure first increases linearly and then decreases slightly. When the suction exceeds 21.8 MPa, the water content of the samples with different dry densities is basically the same. In the low suction range, the water content increases with the decrease of the dry density. During the wetting process, the interlayers of the bentonite absorb water layer by layer, forming no more than two layers of adsorbed water. According to the wetting curves with different dry densities, the contents of the adsorbed and capillary water are calculated using the T₂ distribution curve. It is found that there is mainly the adsorbed water in the bentonite, with a small amount of capillary water (< 5%). The analysis shows that water is first adsorbed to the interlayer region, and the swelling pressure increases linearly. When a small amount of the capillary water is formed, particle slip leads to a decrease in the swelling pressure. Therefore, the swelling pressure evolution during the wetting process is controlled by interlayer hydration under high suction and affected by changes in pore structure under low suction.