Abstract: To investigate the mechanical behavior and fracture characteristics of different types of carbonate rocks under ultra-deep high-temperature and high-pressure (HTHP) coupling conditions, and to provide a mechanical basis for the safe and efficient development of ultra-deep oil and gas resources, this study focuses on three carbonate formations: the Sinian Dengying Formation dolomite in the Sichuan Basin, the Ordovician Yijianfang Formation limestone in Shunbei, and the Cambrian Lower Qiulitage Formation dolomite in Shunbei. Using a self-developed HTHP triaxial rock mechanics testing system, triaxial compression tests were performed under coupled conditions of 150 ℃ and confining pressures ranging from 0 to 230 MPa. Combined with high-energy accelerator CT scanning and three-dimensional reconstruction techniques, the stress-strain characteristics, evolution of mechanical parameters, brittle-ductile transition behavior, and failure modes of the rocks were systematically analyzed. The results indicate that as confining pressure increases, the plasticity of carbonate rocks gradually increases, transitioning from brittle stress drop to strain hardening, with significant differences in transition thresholds among the three formations. The Dengying Formation dolomite remains brittle even at a confining pressure of 220 MPa, exhibiting a distinct stress drop upon reaching peak strength. In contrast, the Yijianfang Formation limestone and Xiaqiulitage Formation dolomite undergo brittle-ductile transition at confining pressures of 60 MPa and 30 MPa, respectively. Both peak strength and peak strain increase with rising confining pressure. The Dengying Formation dolomite exhibits significantly higher peak strength than the other two formations, showing greater sensitivity to confining pressure while having the lowest peak strain. The elastic modulus and Poisson′s ratio exhibit stage-dependent variations with increasing confining pressure: the elastic modulus initially increases rapidly and then stabilizes, while Poisson′s ratio first increases, then decreases, and eventually stabilizes. Among the three rock types, the Dengying Formation dolomite shows the most pronounced variations, whereas the Lower Qiulitage Formation dolomite exhibits the least. Regarding failure characteristics, under low confining pressure (especially uniaxial conditions), tensile fractures dominate, with complex and well-connected crack networks. As confining pressure increases, fracture complexity decreases, and the failure mode gradually transitions to shear failure or plastic flow; under sufficiently high confining pressures, strain hardening occurs, with failure manifesting only as localized internal damage. The transition in failure modes is influenced by lithology, natural structure, and mineral composition. This study reveals the distinct mechanical responses of three carbonate rock types under HTHP coupling conditions, providing key mechanical insights for reservoir stimulation and wellbore stability assessment in ultra-deep energy development.