Model tests are carried out on the large-diameter single pile and the conventional long pile under lateral cyclic loads in clay, which aims to investigate the behavior of pile foundations of offshore wind turbines under the action of cyclic loads from wind and wave. Based on the backbone by API and Masing rules, a cyclic p-y curve is formed, where p is load and y is displacement. The method is presented to calculate the cyclic degradation of a pile subjected to lateral cyclic loading, using a typical clay model in the literature, which modifies the undrained shear strength by accumulated absolute plastic shear strain. By relating cycle numbers and cyclic amplitudes of lateral load to the cyclic degradation of soil around pile, the method can consider various types of lateral cyclic load in offshore projects. The results by model tests and theoretical calculations demonstrate that, under the same cyclic load, the cyclic resistance capacity of a large-diameter single pile with larger stiffness is better than that of a conventional long pile. For the design of a large-diameter single pile foundation of the offshore wind turbine, it is more rational to consider the post cyclic lateral capacity of the foundation with the residual strength of clay.