As a promising sustainable building material, alkali-activated concrete (AAC) is subjected to complex stresses. However, current investigations on the triaxial compression performance of AAC have restricted confinement ratios and do not account for specimen strength effects. In this work, conventional triaxial compression tests were conducted for AAC to investigate the damage mode, mechanical properties, constitutive model and failure criterion under confining pressure. The confinement ratio range is 0-1.4. The results show that the damage mode of AAC transforms from brittle to pseudo-ductile as the confining pressure increases. The triaxial compression strength increases significantly with AAC strength, attaining the deformation capacity with lower strength. A triaxial compression constitutive model available for Portland cement concrete (PCC) can substantially overestimate the axial strain of AAC at peak stress. Based on the experimental results, the triaxial compression constitutive model and failure criterion for AAC were proposed.