The geomembrane impervious barrier of hydraulic structure engineering is usually designed on basis of results of uniaxial tensile test. But geomembrane is actually in plain stress state in practical project. The principal stresses are in biaxial stress state. Two self-developed biaxial tensile devices and corresponding test methods were employed to investigate the mechanical property of geomembrane which is used as the surface barrier of high rock-fill dam. Several groups of tests with different biaxial tensile conditions were carried out for several geomembrane usually used in rock-fill dam. Then the experimental results were analyzed in contrast to that of conventional uniaxial tension tests. The mechanical property include tension strength, elongation, elastic modulus, Poisson’s ratio and true stress-strain relationship under different biaxial tension states were obtained. That revealed the stress-deformation properties and failure mechanism of geomembrane in actual working condition for rock-fill dam. The test results can provide some theoretical foundations and basis for the design and numerical analysis of geomembrane surface barrier of rock-fill dam.