Abstract: The buffer material plays a crucial role in the long-term safety of a high-level radioactive waste repository as it serves as a defense between the waste container and the host rock. To investigate the long-term performance of Gaomiaozi (GMZ) bentonite under repository conditions, based on the China-Mock-up test at engineering scale, the finite element code LAGAMINE is established to establish the THM numerical model. The complex boundary conditions and material properties involved in the test are considered, and the simulated results of temperature, relative humidity and swelling pressure are compared with the experimental ones over the past five years. The temperature in the China-Mock-up test is found to be periodic and linear, primarily influenced by the room temperature, which is accurately captured by the proposed model. Furthermore, the overall variation of the relative humidity at different locations is well reproduced. A desaturation-saturation process is observed in proximity to the heater, while it does not occur far from the heater. Additionally, the simulated results exhibit good agreement with the recorded data, effectively reflecting the increasing trend of stress over time. Besides, the interaction between temperature, humidity and stress of bentonite under the coupled THM conditions is achieved. These findings provide valuable insights into the long-term behavior of buffer materials in a repository environment and serve as an important reference for further understanding.