Post-cyclic undrained strength degradation happens to the saturated soft clay under wave cyclic loading for the excess pore pressure produced, resulting in the significant loss of bearing capacity of the breakwater. Based on the development model of pore pressure and the rule of shear strength of normally consolidation soft clay, the dynamic degradation law of soft clay strength is derived considering the effect of initial static deviator stress. Based on the dynamic degradation law and Mohr-Coulomb yield strength, the numerical development and dynamic calculation process for post-cyclic undrained strength degradation is implemented into software ABAQUS. Numerical simulations of the dynamic and static triaxial tests of Tianjin soft clay are conducted using the proposed dynamic finite element method. Results show that the development curve of maximum excess pore pressure and values of post-cyclic undrained strength predicted by the finite element method are acceptable compared with tests. Moreover, the proposed method can predict the process of post-cyclic strength degradation.