To investigate the long-term creep behavior of deep granite in geological repositories for high-level radioactive waste under coupled temperature and stress conditions, a full-time temperature–stress coupled creep damage constitutive model (TSD model) was developed. The model incorporates a temperature-induced damage factor \({D}_{T}\) , a stress-induced damage factor \({D}_{S}\) , and a coupled damage factor \({D}_{TS}\) . Based on the classical Nishihara model, the elastic, viscous, and viscoplastic components are modified to better capture creep behavior under complex thermo–mechanical conditions. An improved Drucker–Prager yield criterion is introduced to account for temperature and stress effects on cohesion and internal friction angle, enabling accurate description under high temperature and triaxial stress. Triaxial creep test data of granite at 23 °C, 50 °C, and 90 °C are used for validation. Model parameters are identified through a hybrid optimization strategy combining global search and local least-squares fitting. The TSD model successfully reproduces the three typical creep stages of granite, achieving excellent agreement with experiments (R² > 0.99), especially in the nonlinear accelerating phase. Results show that increasing temperature significantly accelerates mechanical degradation, reduces cohesion and internal friction angle, and leads to loss of shear strength. This study provides a theoretical basis for evaluating the long-term stability of host rocks in deep geological disposal of high-level radioactive waste and offers a framework for extending the model to broader temperature ranges and complex environments such as seepage and chemical coupling.
