Based　on　the　offshore　ground　motions　recorded　in　California,　U.S,　and　the　Sagami　trough　zone,　Japan,　the　offshore　ground　motions　time-frequency　distribution　features　were　analyzed　by　the　Hilbert-Huang　transform　method.　The　acceleration　spectrum　and　dynamic　amplification　factor　β　spectrum　were　compared　with　the　design　spectra　in　relevant　seismic　design　standards.　The　research　results　indicate　that　the　offshore　ground　motions　contain　a　wealth　of　long　period　component　sand　that　the　long　period　and　ultra-long　period　components　were　dominant　in　frequency　range.　The　normalized　marginal　spectrum＇s　energy　distribution　was　correlated　with　epicenter　distance.　The　horizontal　offshore　ground　motion＇s　intensity　was　much　greater　than　that　of　the　vertical　ground　motion.　Moreover,　findings　show　that　the　dynamic　amplification　factor　β-values　of　most　offshore　ground　motions　are　greater　than　the　current　platform　value　of　seismic　design　standards　used,　which　means　that　their　long-period　components　are　much　larger　than　the　seismic　design　standards(i.e.,　it　is　risky　to　use　offshore　facility　seismic　design　by　land　seismic　design　standards).　In　view　of　these　results,　there　are　many　engineering　structures　such　as　long-span　bridges　and　oil　and　gas　platforms　whose　long　period　components　of　offshore　ground　motions　warrant　more　attention　in　offshore　engineering　structure　seismic　design.　©　2018,　Editorial　Office　of　Journal　of　Vibration　and　Shock.　All　right　reserved.