Abstract: For the surrounding dam plain reservoirs that use a horizontal anti-seepage scheme, the leakage of reservoir water, rise in groundwater table and other reasons lead to the pore gas in unsaturated soil under the geomembrane gather, rise and change into pressure, which can cause geomembrane air expansion with local swelling. According to its characteristics, the air expansion deformation of geomembrane is simplified as the spherical bulging deformation under ring-restrained conditions. A set of test equipment with four different diameters is developed to study the characteristics of air expansion deformation of geomembrane under ring-restrained conditions, and the influence of the diameter of test equipment on burst pressure of geomembrane is analyzed. The test equipment with outside diameter of 20 cm is selected as the basic equipment of air expansion of geomembrane. A test is conducted using the selected equipment, and the rules of air expansion deformation and failure of geomembrane are summarized. The conclusions are drawn as follows: (1) The stress-strain curves of air expansion deformation of geomembrane can be divided into four stages: linear deformation, strengthening deformation, weakening deformation and bursting break. (2) The air expansion deformation is not uniform, the mechanism of air expansion failure is the tension failure, and the failure mode is a great circle curve through the vertex of spherical cap or a small round curve which does not pass through the vertex of the spherical cap. (3) For the geomembrane with thickness of 0.35 ~ 0.6 mm, the burst pressure of air expansion is 0.08 ~ 0.12 MPa, and the air expansion crown height is 4.6 ~ 5.5 cm. (4) For the geomembrane wiht thickness of 0.35 ~ 0.6 mm, the air expansion elongation is 17.6% ~ 18.5%, which is far less than the unidirectional tensile elongation of geomembrane.