Modified suction caissons (MSCs) acting as offshore wind turbine foundations will generate the accumulated rotation under cyclic loading resulted from waves.The accumulated rotation and the range of soil deformation around the MSC under long-term cyclic wave loading were studied using 3-D numerical simulations.The Morison equation was adopted to calculate the wave loadings.It was found that the MSC accumulated rotation increases linearly with the increase of the logarithm of cyclic number.The normalized expression was proposed to reflect the relationship between the accumulated rotation and cyclic number.The soil deformation range around the MSC increases when increasing the cyclic number and loading amplitude.It can also be concluded that the accumulated rotation increases rapidly with this change of excess pore pressure in the first 4000 cycles.The responses of the MSC to wave and wind loads were also investigated.Results show that the accumulated rotation of the MSC under both wave and wind loadings is larger than that under the wave loading only.