Abstract: The non-thermal-bonding composite geomembrane separately laid by geomembrane and geotextile is adopted in the horizontal anti-seepage control schemes of Datun Reservoir on the Eastern Route of the South-to-North Water Diversion Project. The reason is that the heat-melt craft is easy to burn the composite geomembrane in the process of production and welding construction, and then to cause hidden troubles. The mechanical model for the air expansion deformation of the non-thermal-bonding composite geomembrane is simplified to the spherical bulging deformation under the ring restraint, and the experiment is accomplished. The conclusions are drawn as follows: (1) The bursting pressure of the non-thermal-bonding composite geomembrane (the thickness of geomembrane is 0.4 mm, the mass per unit area of geotextile is 250 g/m²) increases slowly with the increase of loading rate. The basic loading rate, i.e., liquid injection rate, is recommended to be 100 mL/min. (2) The unit tensile force and strain curve can be divided into four stages: linear, yield, strengthening and bursting. Its overall shape is similar to that of geotextile. The curve shape is similar to that of the geomembrane, which has greater influence during the very short linear and yield stages. In the strengthening stage, the curve shape is more like that of the geotextile and is mainly determined by the geotextile. (3) The expansion and bursting pressure is determined by both the geomembrane and the geotextile. In the initial stage of the deformation, the geomembrane bears more internal pressure. Then the geotextile gradually bears more and more internal pressure and plays a decisive role until failure. The bursting pressure is 1.51 MPa, and the bursting height is 31.5 mm. (4) The elongation at break of expansion is 25.3%, which is mainly determined by the geotextile. There are two types of failure patterns: incomplete failure and complete failure, caused respectively by the non-uniform deformation and uniform deformation of the geotextile.