The primary objective of this paper is to study the seepage behavior of the cutoff wall and the working mechanism of the composite geo-membrane applied in the second stage cofferdam of the Three Gorges Dam. The mixture proportion of the plastic concrete applied in the cutoff wall is researched and a recommended proportion is put forward. The difference between the composite geo-membrane and single geo-membrane concerning mechanical strength and permeability is detected and reasons which lead to the difference are found. The test results indicate that the mechanical characters of the composite geo-membrane have been improved remarkably. It is demonstrated that the deformation ratio plays an important role in terms of the permeability by bursting test. The behavior of the composite geo-membrane and the effect of the composite materials in different work-stage are studied and the influence of the elasticity of the composite materials on the practical application of the composite geo-membrane is analyzed. It is found that the strength is not just the superposition of the strength of composite materials and membrane.