Offshore wind turbines are often supported on large diameter monopole foundation suffering various complex external loadings, especially, wind and wave loads. These long-term cyclic loadings cause the monopile foundation to have accumulated deformation and changes in its lateral stiffness, they have important effects on offshore wind turbines’ safety. For considering the effects of long-term cyclic loadings on offshore monopole foundation, the current deign code is to use the p-y curve method adopted in the American Petroleum Institute Standards. The p-y curve is obtained from field tests with less than 200 cycles loads and piles’ diameter range up to 0.6 meters, and may not be applicable to monopile with large diameter and subjected to much more cycles loading. Here, an overall 3-D FE model was built to study displacements of an offshore wind turbine on a monopile in a sandy seabed under long-term cyclic lateral loading. A simplified model to account for the influence of long-term cyclic loading on the soil surrounding the pile was adopted based on the model test results in the literature. A real wind turbine on a monpile foundation was taken as a study object, the effects of long-term cyclic loading on the foundation’s natural frequencies and the pile’s deformation characteristics were analyzed in detail. The results provided a reference for the design of offshore wind turbine’s monopole foundation.