At　present,　the　numerical　analysis　of　the　monopile　foundation　of　an　offshore　wind　turbine　subject　to　complex　environmental　loads　is　a　major　challenge　in　engineering　design.　Based　on　the　concept　of　the　macroelement　model,　this　paper　proposes　a　straightforward,　speedy,　and　accurate　numerical　tool.　Firstly,　the　numerical　limit　analysis　method　is　employed　to　investigate　the　three-dimensional　failure　envelope　surface　of　a　large-diameter　monopile　in　sand,　and　the　analytical　formula　of　the　three-dimensional　failure　envelope　surface　is　obtained.　Then,　using　the　three-dimensional　failure　envelope　and　combining　the　basic　principles　of　the　hypoplastic　theory,　a　hypoplastic　macro-element　of　a　large　diameter　monopile　foundation　in　sand　is　proposed.　The　performance　of　the　macroelement　model　is　calibrated　by　comparing　it　to　centrifuge　test　results　of　a　small-diameter　pile　and　three-dimensional　finite　element　results　of　a　large-diameter　monopile　under　cyclic　loading.　Finally,　the　macroelement　model　is　used　to　analyze　the　performance　of　the　OWT　large-diameter　monopile　foundation　under　wind　and　wave　loads.　The　results　indicate　that　the　macro-element　model　can　precisely　reproduce　the　performance　of　large-diameter　monopile　foundations　under　cyclic　loading,　and　can　effectively　and　accurately　solve　the　complex　pile-soil　interaction　problem　of　OWT　large　diameter　monopiles　under　cyclic　loading.