To meet the efficiency demands of ultra-supercritical power plants, TP347H austenitic stainless steel has been widely used in high‑temperature components such as superheaters and reheaters owing to its good creep and oxidation resistances. However, prolonged exposure to harsh service environments leads to notable material degradation. Aluminide coatings effectively enhance the high-temperature oxidation resistance of TP347H austenitic stainless steel; however, their influence on the high-temperature mechanical properties of steel, particularly the mechanisms governing tensile deformation and fracture behavior, remains poorly understood. To explore the synergistic evolution and mechanical mismatch of the “brittle coating–ductile substrate” system during high-temperature tensile deformation, as well as its failure under coupled thermomechanical coupling, the tensile properties of uncoated (TP347H) and coated (TP347H-C) specimens were systematically evaluated from room temperature (25 °C) to 630 °C. Comparative tensile testing was complemented by microstructural and fractographic analyses to elucidate the effect of the aluminide coating on high-temperature performance. The results revealed that the coating underwent microstructural evolution at elevated temperatures owing to elemental interdiffusion, transforming from an initial distinct three-layer structure into a two-layer configuration accompanied by markedly intensified oxidation of the outermost layer. The coating caused a moderate reduction in tensile properties. At room temperature, the coated specimen exhibited reductions of approximately 20 MPa in tensile strength and approximately 2.5% in elongation compared to the uncoated substrate. This degradation became more pronounced with increasing temperature, reaching approximately 60 MPa in tensile strength and 6% in elongation at 630°C. Fractographic analysis indicated that, although the coating did not alter the macroscopic fracture mode, it notably affected the evolution of microscopic damage. At room temperature, the intrinsically brittle coating fractured during the early stages of plastic deformation, forming a network of microcracks. As the testing temperature increased, the substrate progressively softened while the coating retained its brittle nature. Consequently, coating cracking became severe, accompanied by partial spallation, and inhibition of necking deformation in the substrate. The deterioration in coating failure was attributed to the increasing mechanical mismatch between the coating and substrate at elevated temperatures. The substrate became ductile while the coating remained brittle, the mismatch in deformation capability increased, causing the coating to crack at low strain levels. The resulting cracks act as stress concentrators and crack initiation sites, ultimately accelerating specimen failure.