Abstract: The swelling behavior of buffer/backfill materials in high-level radioactive waste (HLW) repository is one of the significant indices evaluating their buffer/backfill properties. Due to the limitations of test apparatus, most of the previous swelling tests are performed under two simple extreme boundary conditions of constant volume (CV) and constant mean stress (CMS), which can not effectively reflect the complex stress-strain state in the repository. A new set of multi-boundary conditions dilatometer is designed to carry out a series of swelling tests under CV, CMS and constant stiffness (CS) boundary conditions. Based on this custom-made test apparatus, the swelling behavior of GMZ buffer/backfill materials is tested under complex boundary conditions. The results indicate that the developed test apparatus has the advantages of simple operation and stable performance, which can effectively stimulate various boundary conditions. The boundary conditions have an important influence on the ultimate swelling indices of samples. More specifically, the relationship between various boundary conditions and the corresponding swelling pressure is CV > CS > CMS, and the relationship between these conditions and the corresponding swelling strain is CMS > CS > CV. The swelling equilibrium limit (SEL) curve can be used as a reference for predicting the ultimate swelling strain and pressure of soils during hydration under complex boundary conditions. The test results have certain reference significance for further understanding the swelling behavior and guiding the design of buffer/backfill materials in HLW repository.