Abstract: The deformation behavior of seepage-control geomembranes in the core walls of rockfill dams is critical to the safety of dam seepage-control systems. However, conventional uniaxial tensile tests fail to accurately replicate the multidirectional stress states to which geomembranes are subjected during actual operation. To clarify the deformation capacity of geomembranes under actual operating conditions, this study integrates material selection challenges for a rockfill dam core wall and conducts comparative uniaxial/multiaxial tensile tests to investigate the mechanical properties of HDPE, LLDPE, and PVC geomembranes commonly used in dam seepage-control systems. The findings reveal that traditional uniaxial tests overestimate the deformability of geomembranes: the yield strains of HDPE and LLDPE under multiaxial tension (1.6%~2.4%) are significantly lower than those under uniaxial tension (13%), while PVC exhibits no distinct yield point. However, PVC geomembrane demonstrates ultimate tensile strains (92.8%~112.4%) that far exceed those of PE geomembranes (34.4%~50.0%). By evaluating deformation adaptability through elastic-stage strain energy density, PVC exhibits the highest elastic strain energy density under multiaxial loading, substantially greater than both PE variants, with thicker PVC geomembranes showing enhanced deformation accommodation capacity. The study demonstrates that elastic strain energy density derived from multiaxial tensile testing can serve as a reliable criterion for assessing deformation performance in seepage-control geomembranes.