Seismic　analysis　of　many　offshore　structures　is　the　three-dimensional　(3D)　water-cylinder　dynamic　interaction.　The　cylinder　is　commonly　modeled　as　a　beam　by　finite　elements.　If　the　3D　infinite　water　layer　is　also　modeled　by　the　finite　elements,　the　high　computational　costs　are　unacceptable　in　engineering　practice.　Therefore,　an　accurate　and　efficient　time-domain　model　is　proposed　to　replace　the　3D　infinite　water　layer　in　the　water-cylinder　interaction　analysis.　Firstly,　based　on　the　frequency-domain　analytical　solution,　the　exact　dynamic　stiffness　relationship　between　the　hydrodynamic　pressure　and　the　structural　displacement　is　constructed　on　the　water　cylinder　interface.　Secondly,　this　relationship　is　transformed　into　a　high-order　approximation　in　time　domain　by　using　the　temporal　localization　method.　Thirdly,　the　high-order　approximation　is　represented　as　a　mechanical　model　system　consisting　of　the　spring,　dashpot　and　mass　elements,　which　is　implemented　into　the　finite　element　software　ABAQUS　by　the　user　element　subroutine.　Finally,　numerical　examples　are　given　to　indicate　the　effectiveness　of　the　proposed　time-domain　model　and　investigate　the　effect　of　hydrodynamic　pressure　on　the　seismic　responses　of　the　cylinder.