The horizontal response of a loaded pile under the sea has attracted more and more attention in recent years. The ultimate horizontal capacity and stiffness of the horizontal response are two principal aspects of a pile response to a horizontal load that require assessment by the designer. This paper is based on the cyclic mobility model and soil-water coupled FEM, reports a study of the relationship of the horizontal load and displacement and the regulation of the stiffness degradation of a rigid pile under horizontal dynamic load. It also accounted for the difference of the horizontal ultimate pile capacity between dynamic and static horizontal load and conclusions are made that may assist in future design and investigation of this phenomenon.