Understanding the evolution of sandstone mechanical behavior under high pressure and high temperature (HPHT) is crucial for the efficient development of ultra-deep tight reservoirs. In this study, triaxial compression tests on ultra-deep core samples and true triaxial hydraulic fracturing experiments on 200 mm × 200 mm × 200 mm sandstone cubes were conducted under HPHT conditions. The brittle–ductile transition behavior and fracture initiation and propagation characteristics of rocks in ultra-deep reservoirs were investigated, and four classical break-down models were employed to evaluate break-down pressures under different stress state and temperature conditions. The results show that, at elevated confining pressures and temperatures, ultra-deep rocks undergo a transition from brittle failure dominated by shear cracks to ductile deformation involving numerous microcracks; pre-peak plastic strain increases markedly, and a pronounced post-peak stress plateau appears in the stress–strain curves, indicating a significant enhancement of overall ductility. Under HPHT conditions, breakdown pressure increases and fracture propagation resistance becomes stronger, and hydraulic fractures tend to exhibit an intermittent “initiation–arrest–reinitiation” propagation pattern, which is unfavorable for the development of a complex fracture network. Comparison of model predictions with experimental and field data further demonstrates that, for reservoirs deeper than 5000 m, thermally induced stresses should be incorporated into break-down pressure prediction. Among the four models considered, the T-H-W model exhibits superior physical plausibility and predictive reliability for ultra-deep tight reservoirs. These findings provide important experimental and theoretical support for optimizing hydraulic fracturing design and enhancing stimulation effectiveness in ultra-deep tight formations.