There is a long time gap between buffer bentonite block preparation and wetting; thus, it is necessary to study the effect of ageing on swelling characteristics. Gaomiaozi (GMZ) sodium bentonite was used as the test material, and curing periods of 0, 7, 15, 30, and 90 days after preparation were chosen, while maintaining constant dry density and water content. The swelling deformation and pressure tests were performed by saturating the specimens with salt solutions in an odometer. Scanning electron microscope (SEM) was also performed. The test results showed that the swelling deformation and pressure of GMZ bentonite decreased with increasing curing time. The decrease was obvious in the early stage, and it tended to be stable in the later stage. At the same curing time, the ageing effect on the swelling property weakened when the TDS concentration increased. The SEM images revealed that upon increasing the ageing time and TDS concentration, the bentonite was hydrolysed and the pores between the aggregates tended to be homogenised. The variance of pores between the aggregates caused by increasing TDS concentration was larger than that caused by increasing ageing time. This is consistent with the conclusion for the swelling force attenuation rate: under the condition of low TDS concentration and short ageing time, the TDS concentration may have a more obvious influence than the ageing time.