Abstract: Aimming at the deformation adaptability of geomembranes used as barriers in the upper reservoir of pumped storage power stations, a set of bubble inflation multiaxial tension testing equipment for geomembranes is developed. The experimental studies are conducted on the influence rules of the key factors including dimensions of specimen clamp, loading rates, and waiting time of deformation on the test results. Subsequently, a test method to simulate the elastic recoverable deformation properties of geomembranes under fluctuating water levels is formulated based on these findings. This method is applied to the comparative researches on the elastic deformation properties of HDPE, LLDPE and PVC geomembranes commonly used as barriers in the upper reservoir of pumped storage power stations. The experimental results indicate that the traditional uniaxial tensile test method has limitations. Under the multiaxial tensile conditions, the ultimate strains of geomembranes are significantly lower than those obtained from the uniaxial tensile tests. Under the multiaxial tensile conditions, the HDPE and LLDPE geomembranes exhibit pronounced yielding at relatively low strain levels, with elastic recoverable strain ranges of only 1% to 3%. In contrast, the PVC geomembranes do not exhibit significant yielding, with elastic recoverable strain ranges exceeding 22.8%. Moreover, thicker PVC geomembranes demonstrate greater elastic recoverable deformation capabilities and higher tensile strengths. In the selection of geomembranes for engineering design, the geomembranes with sufficient elastic recoverable deformation capabilities should be chosen based on the maximum deformation value of the seepage prevention structures.