The connection effectiveness between cutoff wall and composite geomembrane of cofferdam, both of which are usually applied to the cofferdam, plays an important role in seepage control. Based on the site investigation of stage II cofferdam of Three Gorges Project, through the stress analysis of geomembrane in seepage control and direct shear tests on interface characteristics between composite geomembrane and gravel, the interaction between geomembrane and gravel is studied. The condition to prevent the geomembrane from being broken is proposed. Three centrifuge tests are performed to reveal the mechanism of the connecting form of cutoff wall and composite geomembrane by measuring the displacements of the cofferdam fillings and cutoff wall and the strain of the composite membrane. The results show that the friction between the composite geomembrane and the cofferdam fillings is so large that the expansion joint reservation cannot work under certain overlying load. Two main influencing factors include the settlement of the weathered sand foundation and the lateral displacement of cutoff wall, both of which play a decisive function in the fracture of the composite geomembrane. Furthermore, an improved method of the connecting form, which made the composite geomembrane not be tensioned, is proposed and validated by centrifuge tests. The improved method provides a reliable evidence for the design and construction of cofferdams.