The　dynamic　responses　of　water-pile-soil　system　subjected　to　horizontal　earthquakes　considering　seabed　flexibility　on　hydrodynamic　pressures　is　investigated　in　this　paper.　The　hydrodynamic　pressure　on　the　pile　is　obtained　by　the　inviscid　compressible　fluid　theory　and　the　absorption　of　pressure　waves　due　to　the　seabed　flexibility　is　modeled　by　using　a　reflection　coefficient　which　is　frequency　independent.　The　horizontal　resistance　of　the　soil　is　obtained　based　on　the　viscoelastic　dynamic　continuum　model.　Based　on　the　analytical　solution　for　the　seismic　responses　of　the　free-field　soil,　the　numerical　model　for　the　seismic　response　of　the　pile　is　developed　by　using　the　finite　element　method.　Parametric　studies　are　further　conducted　to　illustrate　the　influence　of　seabed　flexibility　on　the　seismic　responses　of　the　pile.