Abstract: A series of biaxial compression tests were conducted to investigate the macroscopic mechanical response and fracture mechanism of porous sandstone under different intermediate principal stress conditions. The stress-strain behavior, mechanical parameters, failure modes, and the influence of intermediate principal stress were systematically analyzed. The results show that the mechanical evolution of porous sandstone under biaxial loading can be divided into four stages: pore compaction, linear elasticity, plastic deformation, and post-peak fracture. A distinct yield plateau was observed during the plastic deformation stage, while a sharp drop in strength and an almost complete loss of residual strength feature in post-peak fracture stage, exhibiting a hybrid ductile-brittle failure behavior. The intermediate principal stress σ₂ plays a crucial role in strengthening the strength parameters of sandstone. When σ₂<24 MPa, an increase in σ₂ significantly enhances the peak stress, elastic modulus, closure stress, and crack initiation stress of porous sandstone. However, as σ₂ continues to increase beyond this threshold, the strengthening effect gradually diminishes. At the lower intermediate principal stress levels, crack propagation is predominantly governed by tensile cracks, whereas at the higher intermediate principal stress levels, the failure mode gradually transitions to overall instability dominated by compressive-shear cracks. Biaxial stress confinement significantly influence the spatial evolution characteristics of cracks in sandstone. Differnt from the isotropic crack propagation under the uniaxial loading, the confining effect of σ₂ in biaxial loading induces preferential crack propagation along the σ₁-σ₂ or σ₂ plane, substantially reducing crack coalescence and penetration perpendicular to the σ₂ direction.