Abstract: A new method of analyzing earth dam deformation during reservoir filling using FEM is developed in this paper. Comparing with Nobari’s method, which is often used in engineering, two aspects are improved: (a) the elasto-plastic constitutive model with two yield surface, which can reflect the deformation characteristics of soils during reservoir filling more realistically, is used instead of Duncan’s model; (b) the increment method is used to calculate the deformation during reservoir filling, which overcomes the defects in the whole amount method.