Due　to　impeller-tower　coupling　and　aerodynamic　damping,　dynamic　characteristics　of　an　offshore　wind　turbine　in　forward-backward　direction　and　those　in　lateral　direction　are　different.　Here,　for　a　5MW　NREL　single　pile　type　offshore　wind　turbine,　through　theoretical　analysis,　differences　of　vibration　mode　and　damping　characteristics　of　the　turbine　between　its　forward-backward　direction　and　lateral　direction　were　explored.　Then,　under　the　two　working　conditions　of　shutdown　and　operation,　using　the　aerodynamic-servo-hydrodynamic-elastic　fully　coupled　method,　influence　laws　of　ground　motion　input　direction　on　dynamic　response　of　offshore　wind　turbine　structure　were　analyzed　to　explore　the　most　unfavorable　direction.　Finally,　effects　of　average　wind　speed　on　the　most　unfavorable　input　direction　of　ground　motion　were　analyzed.　The　results　showed　that　the　input　direction　of　ground　motion　may　significantly　affect　the　mud　surface　bending　moment　and　tower　top　displacement　amplitude　of　offshore　wind　turbine　support　structure;　under　the　operation　working　condition,　the　lateral　direction　is　the　most　unfavorable　ground　motion　input　direction　under　action　of　strong　earthquake,　the　forward-backward　direction　is　the　most　unfavorable　ground　motion　input　direction　under　action　of　weak　earthquake.　©　2021,　Editorial　Office　of　Journal　of　Vibration　and　Shock.　All　right　reserved.