A　model　of　added　mass　representation　for　a　flexible　elliptical　cylinder　vibrating　in　water　is　presented　to　simulate　the　fluid-structure　dynamic　interaction.　Based　on　the　formula　of　the　hydrodynamic　pressure　on　elliptical　cylinder,　the　free　vibration　equation　for　an　elliptical　cylinder　in　water　is　given.　Further,　the　analytical　solutions　of　the　natural　frequencies　and　mode　shapes　for　the　elliptical　cylinder　vibrating　in　water　are　derived.　The　expression　for　calculating　the　seismic　response　of　the　cylinder　surrounded　by　water　is　obtained　by　the　mode　superposition　method.　Using　the　added　mass　representation,　the　formula　for　calculating　the　fundamental　frequency　of　the　elliptical　cylinder　in　water　is　proposed.　A　simple　formula　for　evaluating　the　natural　frequency　is　proposed　by　curve　fitting　method.　The　mathematical　expression　of　the　formula　is　very　simple　and　it　is　suitable　for　engineering　application.　©　2019,　Nanjing　Univ.　of　Aeronautics　an　Astronautics.　All　right　reserved.