The mechanical properties and features of two main components of the Three Gorges Project's Phase II cofferdam are obtained based on sampling and experiments and the deformation law is found. Based on the experimental results and measurement, the 3 parameters rheological model is put forward, which can describe not only the behavior of creep, but also the feature of stress relaxation and that corresponding with the viscoelasticity of majority materials for the cofferdam material. The model meets the viscoelastic mechanical properties of most materials. Based on the Duncan E- μ constitutive model, the FEM analysis is programmed. According to the measured deformation data of the cofferdam, the deformations of cofferdam and cutoff wall are calculated. The computation result shows that under the high head in flood season and once load, the instantaneous deformation has the following law: the vertical displacement of most cofferdam elements is settlement, only a few are upward displacement. The structural deformation of the cutoff wall is consistent with that of the cofferdam, which proves that the cofferdam can operate safely under high head.