Offshore　wind　turbine　(OWT)　structures　are　subject　to　wave,　wind,　and　seismic　loading.　Due　to　the　cyclic　nature　of　these　loads,　OWT　foundations　can　be　vulnerable　to　cumulative　deformation　and　liquefaction　triggered　by　waves　and　earthquakes.　The　effects　of　cumulative　deformation　and　liquefaction　on　the　monopile-supported　OWT　are　not　fully　appreciated.　This　paper　develops　a　three-dimensional　numerical　model　for　analyzing　the　seismic　performance　of　large　monopile-supported　OWT　under　the　long-term　effect　of　cyclic　loading.　The　numerical　model　was　established　employing　FLAC3D　and　utilizing　SANISAND　constitutive　model　to　simulate　the　soil　behavior.　The　numerical　model　was　validated　by　comparing　its　predictions　with　the　results　of　dynamic　triaxial　tests　and　centrifuge　tests.　A　simplified　densification　and　subsidence　site　model　was　integrated　into　the　numerical　model　to　facilitate　considering　the　long-term　effect　of　cyclic　loading.　The　numerical　model　was　then　used　to　conduct　a　comprehensive　study　to　evaluate　the　influence　of　long-term　cyclic　loading　on　the　natural　frequency　and　seismic　response　of　OWT　structure.　The　results　demonstrated　that　the　densified　subsidence　zone　around　monopile　increased　the　liquefaction　resistance.　However,　the　horizontal　displacement　of　pile　and　the　response　acceleration　of　tower-top　increased　due　to　soil　subsidence　around　monopile.