The　analytical　method　was　adopted　to　investigate　the　earthquake-induced　hydrodynamic　pressure　on　a　circular　cylinder　surrounded　by　two-layer　fluid.　Based　on　the　radiation　theory,　the　analytical　expressions　of　hydrodynamic　pressure,　hydrodynamic　force　on　a　rigid　circular　cylinder　were　obtained.　Then,　the　dispersion　relation　for　radiation　problem　in　a　two-layer　fluid,　and　the　added　mass　and　damping　coefficients　on　the　circular　cylinder　in　the　case　were　analyzed.　The　earthquake-induced　hydrodynamic　forces　on　the　circular　cylinder　in　a　two-layer　fluid　were　furtherly　investigated.　It　is　founded　that　the　hydrodynamic　pressures　on　the　circular　cylinder　include　not　only　the　surface-wave　and　perturbing　wave　modes,　but　also　the　internal-wave　mode.　This　is　quite　different　from　the　case　of　a　single-layer　fluid.　However,　the　results　indicate　that　the　internal-wave　mode　has　little　influence　on　the　hydrodynamic　forces　on　the　circular　cylinder　in　two-layer　fluid.　Lastly,　seismic　responses　of　the　flexible　cylinder　in　single-layer　and　two-layer　fluid　were　studied.　Some　examples　show　that　the　seismic　responses　of　the　cylinder　in　two-layer　fluid　are　much　larger　than　these　in　single-layer　fluid.