Abstract: In underground excavation and associated stress adjustments, the surrounding rock mass typically experiences non-hydrostatic and deviatoric multi-axial stress conditions. Among the three principal stresses, the intermediate principal stress (σ₂) has been increasingly recognized as a critical factor governing the deformation behavior, crack evolution, and failure mode of brittle geomaterials. Nevertheless, the stage-dependent regulatory mechanism of σ₂ across different stress regimes remains insufficiently established, particularly under physical biaxial loading conditions that directly capture excavation-induced stress redistribution. To address this gap, comprehensive biaxial compression tests covering σ₂ levels from 0 to 36 MPa were conducted on sandstone specimens using a self-developed biaxial static–dynamic loading system. Acoustic emission (AE) monitoring and three-dimensional digital image correlation (3D-DIC) techniques were simultaneously employed to capture multiscale responses ranging from internal microcracking to macroscopic failure. The mechanical behavior, AE multifractal spectrum characteristics, RA (Rise time/amplitude)–AF (Average frequency)-based crack-type discrimination, and evolution of DIC-derived apparent strain-dominant zones were integrated to systematically quantify the influence of σ₂ on the progressive failure process. The results reveal a pronounced stage-dependent σ₂ effect, with approximately 20 MPa identified as the critical transitional stress level at which the fracture mechanism undergoes a fundamental shift. In the low σ₂ regime of 4–16 MPa, sandstone exhibits compaction-enhanced strengthening, simple and localized crack propagation, and a predominant shear fracture mode, which is consistent with a structurally stable microcracking process. However, when σ₂ reaches 20 MPa, multiple indicators—including the abrupt increase in the peak strength deviation, marked widening of AE multifractal spectra, and reversal of RA–AF crack-type proportions—show synchronous transitions. At this stage, tensile cracks exceed shear cracks for the first time, indicating a shift from a shear-dominated failure regime to a tensile–shear interactive fracture mechanism. This transition is corroborated by the DIC strain-field evolution, which shows that the localized shear-dominant strain band observed at low σ₂ evolves into a planar tensile–shear composite strain-dominant region at 20 MPa. As σ₂ increases further to 24–36 MPa, the sandstone exhibits a more complex mixed cracking pattern, characterized by the coordinated propagation of tensile and shear fractures, as well as the formation of large-scale tensile–shear conjugate structures. Therefore, the high σ₂ regime can be considered a mechanically enhanced but structurally unstable failure stage. Together, the consistent transitions observed across mechanical curves, AE multifractal indicators, crack-type discrimination, and strain-field evolution strongly support the identification of 20 MPa as the stage-dependent turning point governing the σ₂ effect. Overall, this study provides experimentally validated insights into the transitional role of the intermediate principal stress in controlling the multiscale fracture evolution of sandstone. These findings contribute to a deeper understanding of the stage-dependent failure mechanisms of brittle rocks under non-hydrostatic confinement and offer practical implications for evaluating the stability and failure risks of rock masses subjected to complex in situ stress conditions.