Abstract The large-diameter single-pile foundation is the main form of offshore wind turbine. It is of great significance to evaluate the horizontal bearing capacity of the pile foundation and the 1st natural frequency of the wind turbine to optimize the design. The numerical analysis method of the strength degradation of soft clay based on non-linear kinematic hardening model was developed. The comparisons with triaxial tests and centrifuge test results showed that the method had reliabilities in analyzing the strength degradation of the soil and interaction of pile-soil system. The method analyzed the effects of cyclic loading on the stiffness of pile-soil system, the bearing capacity of pile foundation and the 1st natural frequency of the wind turbine. The results showed that the effect of cyclic load amplitude was greater than the number of cycles when the number of cycles was small. The bending stiffness and flexural bearing capacity of the pile-soil system were attenuated more than the horizontal stiffness and the lateral bearing capacity. Due to the limited degree of stiffness attenuation, the overall first-order natural frequency of the wind turbine was not significantly reduced.
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