Abstract: The elastoplastic dynamic constitutive model plays an important role in the design of embankment or foundation under cyclic loading. The main challenge in the design of geotechnical engineering is to establish a dynamic constitutive model which is practical and universal. Based on the critical state theory, an elasto-plastic constitutive model is established to characterize the cyclic degradation, hysteresis and deformation accumulation of saturated soft clay under cyclic loading. In the p’-q space, parameters n and ξ are introduced into the Cam-clay model, and the Cambridge bounding surface equations, which can reflect the shape of water drop and similar ellipse, are proposed. Based on the triangle similarity principle, the mapping relationship between the loading surface and the yield surface is established, and the cyclic degradation modulus field is formed. Nine parameters in the model are determined by the routine laboratory tests. To verify the correctness of the model, the dynamic triaxial tests and numerical simulations are carried out under the isotropic cyclic loading and the anisotropic cyclic loading. The stress-strain relationship and stress paths of the saturated soft clay are analyzed. The research results demonstrate that the test results are basically consistent with the predicted ones by the model, and the proposed model can correctly analyze the cyclic degradation, nonlinearity, strain accumulation and hysteresis characteristics of saturated soft clay.