Abstract: Based on the dynamic governing equations of unsaturated soil, the horizontal transient response of a single pile in unsaturated soil was analyzed. By using the Laplace Transform and method of normalization, the horizontal wave equations of transform domains in unsaturated soil were established. By the method of separation variables and the operator-decomposition theory, the displacement solutions of unsaturated soil in the Laplace Transform Domain were obtained. To solve the soil-pile coupling system, the horizontal vibration equations of pile foundations and the continuity conditions were both adopted, and then the Laplace Transform's expressions of the displacement and internal forces of piles were obtained. By numerical calculation of Laplace's inverse transforms, the effects of the degrees of saturation, pile-soil modulus ratios and load duration on dynamic responses of piles were studied. The results showed that:the degrees of saturation influenced the displacement and internal forces of piles to some extents. The larger pile-soil modulus ratios, the influence of degrees of saturation on piles more significant. The longer the duration of impact loads, the greater the pile displacement, and the more the lag of the peak displacement.