Abstract: Strength reduction may take place for the littoral soft clay under the action of wave loading. As a result, the bearing capability of soft clay foundation will become significantly lower, and the stabilization of the structures will be affected. By performing cyclic triaxial tests on muddy-silty clay taken from Yantai littoral zone, variation of pore water pressure and weakening of undrained shear strength of clay subjected to different confining pressures, deviator stresses, dynamic stresses and cycle numbers are studied. According to the laws of the development of pore pressure, a model for calculating pore pressure is proposed. In addition, by using the theory of quasi-overconsolidation and combining the pore pressure model and the laws of undrained shear strength, a formula for strength weakening which contains confining pressures, deviator stresses, dynamic stresses and cycle numbers is put forward. The relevant experiment is done to verify the formula. Then it is applied to the dynamic numerical model of a breakwater in Yantai Port. From the experiment to the numerical model, a dynamic calculation method considering the effect of strength weakening which is convenient to be used in engineering is proposed completely.