Abstract: In the geomembrane anti-seepage structure, pore networks formed between the particles of the porous medium cushion layer. Under the influence of hydrostatic loading, the upper geomembrane undergoes liquid-phase expansion deformation due to compression. The mechanical properties of this bulge play a crucial role in ensuring engineering operation safety. In this study, experimental research on the swelling mechanical properties of three types of PVC-P GMB was conducted using a GMB bursting strength tester and 3D digital image related technology (DIC). The distribution and evolution trend of field strain within the failure range were analyzed based on the measured field displacement and swelling pressure of the entire process of swelling deformation samples. Furthermore, the distribution of field stress was investigated using Neo-Hookean hyperelastic strain potential and a three-level Prony mathematical model, establishing the stress-strain relationship in the failure region. The findings demonstrate that the stress-strain relationship in the region of swelling deformation and failure provides a more accurate reflection of the mechanical properties of PVC-P GMB, while current standard analysis methods tend to overestimate its deformation capacity. The zero shear strain distribution in PVC-P GMB exhibits a "crescent rib" pattern from the central region towards the outer boundary, which governs the shape of swelling deformation and failure. The second strength theory is deemed suitable for evaluating the mechanical properties of PVC-P GMB.