An endochronic dilation model for marine soils on the basis of Gibbs free energy and stress intrinsic time scale is presented and used to found pore water pressure(pwp) equation to predict pwp under undrained uniform or non-uniform cyclic ocean-wave loadings. The equation is used for performing numerical simulation and prediction to some previous and writer’s cyclic test data. It is shown that equation provides a reasonable description for pwp under undrained cyclic loadings. The model is concise and its basic parameters are few and easy to decide. It is convenient to apply the model to the analysis of liquefaction potential under ocean-wave loading. The mechanism the progressive increase of pwp during undrained cyclic loading is also illustrated well by the model.