The high earth and rockfill cofferdam of Three Gorges Project employs plastic co ncrete diaphragm walls as seepage barrier. In view of the importance and technic al difficulty of the cofferdams, the behavior of plastic concrete and the diaphr agm walls within the cofferdams was studied. Plastic concrete samples taken from concrete mixer in the dam site were tested by using large triaxial testing appa ratus in Tsinghua University. Mechanical properties and parameters of Duncan-Ch ang nonlinear elastic model were obtained. Test results indicated that, comparin g with ordinary concrete, the mate rials have the features of low modulus of deformation and favorable impermeabili t y. The analysis of stresses and deformations of the diaphragm walls was performe d by means of finite element method (FEM), using parameters obtained from the re sult of triaxial tests. Calculation results were discussed.