Abstract: As the core hot-end component of heavy-duty gas turbines, turbine blades' structural integrity directly determines the operational safety and reliability of the unit under extreme service conditions. To address the failure issue of first-stage turbine blades in a certain type of heavy-duty gas turbine under complex multi-field coupled loads, this paper proposes a comprehensive evaluation method that combines fluid-thermal-structure coupled simulation with computational fracture mechanics. By building a high-fidelity multi-field coupled model that considers the combined effects of mainstream gas, internal cooling, and centrifugal loads, it achieves high-precision reconstruction of 3D temperature and stress fields under real service conditions. The study shows that influenced by local cooling insufficiency, local hot spots form on the suction surface at the blade tip, with the maximum temperature reaching 1377.87 K; under the dominance of combined centrifugal and thermal stresses, the maximum equivalent stress is 777.30 MPa, concentrated in the blade root tenon area, which closely matches the actual failure location. On this basis, stress intensity factors at the tips of cracks of different sizes were systematically extracted, revealing the mechanical driving mechanism of crack initiation and evolution; the Paris formula was used to predict the fatigue crack growth rate, and it matched the experimental values closely, confirming that the model is reliable.. This study achieves cross-scale integration of macroscopic multi-field reconstruction and microscopic damage mechanisms, providing a theoretical basis for the long-life design and safety assurance of hot-end components.