<p>This study investigates the mechanical behaviour and brittle-ductile transition of porous sandstone. Conventional triaxial compression tests were analysed, the 3D strength characteristics were quantified using a modified Hoek-Brown criterion, and an elastic-plastic constitutive model with double yield surfaces was developed based on generalised plasticity theory. The brittle-ductile transition pressures were determined for different porosity sandstones, and a brittleness index was derived to evaluate confinement effects on failure mechanisms. The 3D strength envelope shows that stress increments at failure decrease with increasing confining pressure, and the octahedral plane evolves toward a rounded triangular shape, reflecting enhanced isotropy. The elastoplastic model reproduces stress-strain behaviour under varying confinements, capturing strain hardening, softening, volumetric dilation, and confinement effects, with results closely matching experimental peak strengths and strains, providing a reliable framework for simulating the macroscopic mechanical response of porous sandstone.</p>