Abstract: To investigate the biaxial compressive behavior and failure criteria of fiber-reinforced high-performance concrete (FRHPC), 80 biaxial compression tests were conducted, considering fiber parameters (volume fraction and aspect ratio) and varying levels of lateral compressive stress. The evolution of compressive strength and deformation characteristics was systematically analyzed. Experimental results reveal that fiber addition significantly alters failure modes: plain concrete mainly exhibits laminar splitting failure; polypropylene fiber-reinforced concrete is dominated by splitting failure with secondary cracks; while steel fiber-reinforced and steel-polypropylene hybrid fiber-reinforced concretes consistently show diagonal shear failure. Steel fibers provide a more pronounced enhancement in biaxial strength than polypropylene fibers, with strength increasing alongside lateral stress ratio—reaching a maximum improvement of 34.78% at a ratio of 0.5 before declining. A failure criterion incorporating fiber parameters was proposed based on the Kupfer strength envelope, which accurately predicts the biaxial compressive strength of FRHPC. In addition, a 2D mesoscale numerical model was developed using ABAQUS/Python, effectively reproducing the stress-strain response and crack evolution under biaxial loading. These findings offer both experimental and theoretical insights into the behavior of FRHPC under complex loading conditions.