Offshore　wind　turbine　structures　are　commonly　subjected　to　cyclic　lateral　loading,　from　waves　and　wind.　The　cyclic　loading　can　cause　an　accumulated　deformation　of　monopile.　This　study　investigates　cyclic　accumulated　deformation　of　offshore　monopiles　by　using　soil　cyclic　shear　tests　and　numerical　simulation.　A　series　of　soil　axial-torsional　bi-directional　cyclic　load　tests　are　conducted　to　obtain　the　cyclic　behavior　of　sand　with　different　relative　densities.　The　test　results　are　analyzed　in　terms　of　soil　secant　modulus　from　accumulated　shear　strain,　and　the　results　are　then　used　to　improve　a　degradation　stiffness　method.　This　improved　method　was　verified　by　comparing　with　a　pile　test.　Finally,　the　cyclic　accumulated　deformation　of　large-diameter　monopile　was　studied　in　loose　over　dense　sand,　medium-dense　over　dense　sand　and　dense　sand　by　using　this　improved　method.　The　results　show　that　the　secant　modulus　decreases　rapidly　in　the　initial　cycles　of　loading,　and　then　the　rate　of　decrease　diminishes　and　remains　below　an　asymptote.　The　depth　of　the　max　bending　moment　increases　with　increasing　the　cyclic　number.　©　2022