Dynamic responses of soil foundation are difficult to analyze, though it is related to kinds of practices. In this paper, dynamic responses of soil foundation under dynamic load were analyzed theoretically. Perturbation method was used to obtain the theoretical solution. Controlling equations were about liquid-solid two-phase media. Flow, function, and potential function were introduced to decouple the controlling equations. Then, perturbation expansion was introduced into the equations of flow, function, and potential function. The responses' characteristics are discussed.