Abstract: The failure of a saturated slope under an intense earthquake ground motion will produce large sliding deformation, which is difficult to analyze using the finite element method. However, the smooth particle hydrodynamics method (SPH method) can deal with large deformation easily. In this study, to develop a new numerical method for calculating the seismic sliding large deformation of saturated slopes, the motion equation for the SPH method is improved by (1) introducing the Rayleigh damping into the smooth particle hydrodynamics and (2) adopting the effective stress constitutive model. To evaluate the effectiveness of the new method at soil element levels, the effective stress path and shear stress-strain hysteresis curve are obtained through the hollow cylindrical torsional shear tests on saturated soil samples, and are compared with the simulated results. The consistent results are obtained to verify the feasibility of the improved method. Finally, the seismic sliding behavior of saturated slopes under different SPH particle densities is analyzed by using the established numerical method and compared with that by the Newmark's sliding block method. It is found that the key parameter of the improved method is the particle density and that the shape of seismic sliding critical surface of saturated slopes determined by the improved SPH method is in good agreement with that of logarithmic spiral slip surface determined by the Newmark's method. Therefore, the improved SPH method can also be used to simulate a large sliding deformation process of saturated slopes under seismic actions.