The connection effectiveness between cutoff wall and composite geomembrane of cofferdam, which are both usually applied in the cofferdam engineering, plays an important decisive role in seepage control. Based on the site invitation of stage II cofferdam of TGP (Three Gorges Project), three centrifuge tests are performed in order to reveal the mechanism of the connecting form of cutoff wall and composite geomembrane through measuring the displacements of the cofferdam the 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 deformation expansion joint can’t 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 on the fracture of the composite geomembrane. Furthermore, an improved method of the connecting form of the cutoff wall and composite geomembrane is proposed and validated by centrifuge testing. The improved method provides a reliable evidence for cofferdam engineering designing and construction.