Abstract: The connection effectiveness between cutoff wall and composite geomembrane of cofferdam, both of which are usually applied in the cofferdam, plays an important role in seepage control. Based on the site invitation of stage II cofferdam of Three Gorges Project, three centrifuge tests are performed in order 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 friction between the composite geomembrane and the cofferdam fillings is so large that the expansion deformation reservation cannot work. Two main influencing factors include the settlement of the weathered sand foundation and the lateral displacement of cutoff wall, which both play a decisive function in the fracture of the composite geomembrane. Furthermore, an improved method of the connecting form, which make 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 the seepage control system of cofferdams.