The long-term cumulative deformation characteristics of offshore wind turbine monopile foundations are key issues that need to be considered in design. However, existing standards do not take into account the impact of the number of cycles and the amplitude of cycles on the cumulative deformation of monopiles. To address this issue, this study conducts numerical analysis and uses a custom field variable subroutine to investigate the cumulative deformation of monopiles with different sizes under different cyclic amplitudes. The specific conclusions are as follows: A threshold for cyclic loading exists in soft soil. When the cyclic amplitude ξ≤0.2, the long-term cumulative deformation of monopiles under cyclic loading can be neglected. However, when the cyclic amplitudes are 0.4 and 0.6, the monopile undergoes significant cumulative deformation, and the deformation increases according to a power function, with the rate of increase slowing down as the number of cycles increases. When the pile length is fixed, the cumulative deformation increases as the pile diameter increases. When the pile diameter is fixed, increasing the pile length effectively reduces the occurrence of cumulative deformation. However, when the cyclic amplitude is 0.4, a pile length threshold exists. Specifically, when L/D≥15, further increasing the pile length will not reduce the cumulative deformation. Under 200 cycles of loading, the cumulative deformation generated by a cyclic amplitude of 0.6 is six times that generated by a cyclic amplitude of 0.4 for the same pile diameter.