Abstract: In the initial storage period of the plain reservoir using geomembrane as anti-seepage materials of reservoir basin, the leakage water resulting from geomembrane defects is easy to induce the pore-air in the unsaturated soil under geomembrane gathering and rising, and the pore-air will lead to air expansion of the geomembrane with local swelling. The mechanism of air expansion phenomenon resulting from the leakage water is analyzed. A special large-scale test instrument for the air expansion of geomembrane is developed. Air expansion tests in seepage area and anti-seepage area are conducted to simulate the air expansion phenomenon caused by the leakage water resulting from geomembrane defects. The conclusions are drawn as follows: (1) when the groundwater level is invariant, there will be no geomembrane air expansion phenomenon during the storage period; (2) in the initial storage period of plain reservoirs, water leaks from the geomembrane defects, intrudes the porous space of sand ditches under geomembrane, seals off exhaust passages, and causes water resistance phenomenon, and then affects pore-air exhaust from sand ditch exhaust system regularly; (3) in the initial storage period of plain reservoirs, water leaks from the geomembrane defects, and after the leakage water meets the underground water, leakage water will squeeze the space of pore-air in unsaturated soil under geomembrane, induce the overall rising of groundwater level in the reservoir area and gathering and rising of pore-air in unsaturated soil, finally, it causes local expansive deformation of the geomembrane, that is, the air expansion phenomenon.