Abstract: 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 host rock. In this paper, to investigate the long-term performance of Gaomiaozi (GMZ) bentonite under repository conditions, we focused on China-Mock-up test at engineering scale and employed the finite element code LAGAMINE to establish the THM numerical model. The complex boundary conditions and material properties involved in the test was considered and the simulated results of temperature, relative humidity, and swelling pressure were compared with the experimental ones over the past five years. The temperature in the China-Mock-up test was found to be periodic and linear, primarily influenced by the room temperature, which was accurately captured by the proposed model. Furthermore, the overall variation of relative humidity at different locations was well reproduced. A desaturation-saturation process was observed in proximity to the heater, while it did not occur far from the heater. Additionally, the simulated results exhibited 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 coupled THM conditions was 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.