Combined wave and current loading is a very common and important situation for sandy seabeds. The dynamic seabed response under combined these loadings, including the change of excess pore water pressure and the stress, is of great significance to the coastal structures design and construction In this paper, based on two-phase u-p theory for saturated soil, a new constitutive model, which is developed from the concept of superloading and sub-loading, is proposed to analyze the dynamic responds in a sandy seabed under the combined wave and current loadings. Firstly, a poro-elastoplastic model for a sandy seabed is established. The seabed soils parameters are obtained from the un-drained triaxial cyclic loading tests. The mechanics of oscillatory excess pore water pressure and residual excess pore pressure due to waves/currents are discussed and the liquefaction depth which changes correspondingly is obtained. A comparison between the dynamic responds of elastic and elatoplastic sandy seabeds to waves/currents loading is presented. Then, a parametric study is carried out to investigate the effects of currents on the pore pressures and liquefaction.