Abstract: Aimming at the deformation adaptability of geomembranes used as barrier in upper reservoir of pumped storage power stations, an equipment of bubble inflation multiaxial tension tests for geomembranes was developed. Experimental investigations were conducted to explore the influence rule of key factors including specimen clamp dimensions, loading rates, and deformation waiting time on the test results. Subsequently, a test method simulating the elastic recoverable deformation properties of geomembranes under conditions of fluctuating water levels was formulated based on these findings. This method was applied to comparative experimental research on the elastic deformation property of HDPE, LLDPE, and PVC geomembranes commonly used as barrier in upper reservoir of pumped storage power stations. The experimental results indicate that the traditional uniaxial tensile test method has limitations; under multiaxial tensile conditions, the ultimate strains of geomembranes were significantly lower than that obtained from uniaxial tensile tests. Under multiaxial tensile conditions, HDPE and LLDPE geomembranes exhibit pronounced yielding at relatively low strain levels, with elastic recoverable strain ranges of only 1% to 3%. In contrast, 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, geomembranes with sufficient elastic recoverable deformation capabilities should be chosen based on the deformation magnitude of the seepage prevention structure.