Offshore　wind　turbines　(OWTs)　are　dynamically　sensitive　systems　and　vulnerable　to　external　excitations　such　as　aerodynamic　and　hydrodynamic　loadings.　These　loadings　may　lead　to　excessive　oscillation　for　the　support　structure　of　OWTs.　Therefore,　it　is　essential　to　evaluate　the　natural　frequencies　of　the　system　to　keep　them　away　from　excitation　frequencies　of　external　vibration　sources.　This　paper　investigates　the　natural　frequencies　and　seismic　responses　of　the　OWT　system.　The　soil-structure　interaction　(SSI)　is　modelled　by　three　coupled　springs　and　fluid-structure　interaction　(FSI)　is　simulated　by　the　added　mass.　Then　a　closed-form　solution　of　the　natural　frequencies　is　derived　to　consider　the　variation　of　cross-section.　Furtherly,　seismic　responses　of　the　system　are　obtained　by　mode　superposition　method.　Effects　of　SSI,　substructure　mass　and　axial　loading　are　finally　investigated.　The　results　indicate　that　substructure　mass　can　significantly　decrease　the　frequencies　of　the　structure　while　axial　loading　plays　only　a　small　part　in　the　process.　SSI　has　an　important　effect　on　the　seismic　responses　of　OWTs.　It　is　essential　to　consider　the　cross-coupling　spring　to　analyze　the　seismic　responses　for　accuracy.　©　Published　under　licence　by　IOP　Publishing　Ltd.