The　safe　and　stable　operation　of　monopile　offshore　wind　turbines　(MOWTs)　under　seismic　and　environmental　loading　is　a　major　challenge　facing　designers.　A　numerical　method　was　developed　for　simulating　the　dynamic　interaction　of　the　soil-pile-structure　system　by　employing　an　elastoplastic　bounding　surface　constitutive　model　that　can　capture　the　cyclic　degradation　of　clay　stiffness　well.　Then,　the　suitability　of　the　method　was　validated　by　comparison　with　the　existing　simulation　results　in　the　literature.　The　dynamic　responses　of　MOWTs　to　seismic　and　environmental　loading　were　simulated　by　employing　the　above　numerical　method.　The　effects　of　soil　plasticity　and　stiffness　degradation　on　the　dynamic　responses　of　MOWTs　were　investigated.　The　influence　of　environmental　loads　on　the　seismic　responses　of　MOWTs　was　also　analysed.　It　was　found　that　the　monopile　experienced　significant　rotation　and　settlement,　and　the　soil　plasticity　and　stiffness　degradation　significantly　increased　the　peak　response　of　the　lateral　deflection,　rotation　angle　and　bending　moment　of　the　MOWT　under　combined　seismic　and　environmental　loads.　The　input　bedrock　peak　acceleration　was　amplified　to　varying　degrees　from　the　monopile　bottom　to　the　tower　top,　and　the　acceleration　amplification　along　the　height　was　the　most　significant　overall　when　considering　the　soil　stiffness　degradation.　Environmental　loads　such　as　winds　and　waves　significantly　increased　the　absolute　values　of　the　seismic　response　profile,　including　the　deflection,　rotation　angle　and　bending　moment,　as　well　as　the　acceleration　amplification　factor.　The　research　results　can　support　the　seismic　design　of　MOWTs.