<p>Fracture is one of the most common defects encountered in laser shock forming of thin foils. In this study, the fracture mechanisms of copper foils during laser shock bulging were investigated experimentally and numerically under different ratios of laser beam diameter (<i>d</i>) to die cavity diameter (<i>D</i>). The failure locations, surface morphologies, and fracture modes predicted by simulation are in good agreement with the experimental results. Three distinct fracture modes were identified: a tensile fracture mode occurring near the loading edge, driven by tensile stress at <i>d</i>/<i>D</i> = 0.5; a shear fracture mode at the die fillet, induced by shear stress at <i>d</i>/<i>D</i> = 1.25; and a mixed tensile-shear fracture mode developing at both the loading edge and the die fillet, caused by combined tensile and shear stresses at <i>d</i>/<i>D</i> = 0.75. Further analyses of the stress, strain, and thickness distributions reveal that fracture initiates in regions exhibiting severe thickness reduction resulting from intense stress concentration. Among the three modes, the shear failure mode exhibited the smallest bulging depth and thickness reduction ratio, indicating that a large <i>d</i>/<i>D</i> ratio or an excessively high forming velocity is unfavorable in laser shock bulging.</p>