Abstract: In the process of establishing the numerical model to study the soil-structure dynamic interaction problem, the wave input method and the simulation of wave propagation behavior within the semi-infinite medium are critical. There are many wave input methods, such as the wave method, method of substructure of artificial boundary and domain reduction method. The domain reduction method has some advantages over the other methods. On one hand, the earthquake input motion is converted to the source excitation motion within the model, thus the absorbing boundary works better. On the other hand, the size of the numerical model is reduced. The domain reduction method is used for saturated soils through the similar formulation, and the effective forces and effective flow are derived. The proposed method is verified by a simple numerical test.