In this study, the effect of ageing on the tensile strength of compacted Gaomiaozi bentonite was investigated under different dry densities and water contents. The dry densities and water contents were maintained constant during curing periods of 0, 7, 15, 30 and 90 days, respectively. The tensile strength was measured using Brazilian tests and the specimens were analyzed through mercury intrusion porosimetry tests. The test results indicate that the tensile strength of Gaomiaozi bentonite decreases significantly in the early stage and tends to stabilize gradually in the later stage with ageing time. At the same water content, the effect of ageing on the tensile strength of the bentonite becomes less significant with the increase in dry density. The effect of ageing on the tensile strength is due to the gradual movement of the water molecules in the soil to the smectite interlayer during the curing periods and the subsequent gradual development of the smectite inter-lamellar water absorption. This process weakens the connection, decreasing the tensile strength. Smectite hydration results in the decrease in the number of macropores and the gradual increase in the number of small pores among the aggregates. Thus, the findings of this study provide a reference for the range of tensile strength of a bentonite block in a nuclear waste repository during storage and placement.