<p>Hydraulic–mechanical fracturing behavior and mechanisms significantly affect the performances of unconventional resource exploitations. However, the dynamic fracturing processes and failure mechanisms of reservoir rocks under triaxial compression conditions (TC) are still well understood. To address this issue, the formation stress state and hydraulic–mechanical coupling fracturing behavior under TC are captured using 4D X-ray CT imaging due to its advantages of temporal continuity mechanism capture ability under the same sample and stress paths. The fluid–solid interactions between the injected fluid and rocks are analyzed, and the overall hydraulic–mechanical fracturing processes and failure mechanisms are revealed using the digital quantitative analysis (DQA) techniques. Results show that the whole hydraulic–mechanical fracturing process can be divided into the stages of lateral pore compression and closure, porosity increase, wall rupture formation and wellbore collapse, lateral microscopic tension fracture formation, macroscopic tension fracture and oblique shear fracture formation, and specimen failure. The initial hydraulic response is seepage-dominated and characterized by the gradual filling of pre-existing pores without fracture connectivity development with a critical water phase ratio of 0.15. The transition to rupture initiation occurs at the water phase ratio of 0.15 – 0.2, which corresponds to effective stress reduction below tensile strength at the wellbore boundary, marking the onset of tensile rupture. The ratio and volume of 3D fractures increase from 0.03 to 0.054 and 3725.90 mm³ to 6802.716 mm³, which implies the formation of a dominant hydraulic pathway under the tensile-dominated structural evolution. When the ratio and volume of 3D fractures, respectively, increase to 0.07 and 8830.46 mm³, the tensile-to-shear transition mechanism controls the coalescence of tensile fractures, and the redistributed effective stress satisfies shear failure under triaxial confinement, resulting in oblique shear fracture formation. These findings are helpful for hydraulic fracturing parameters and designs during deep reservoir energy and resource exploitations, such as shale oil/gas and geothermal energy.</p>