Based　on　elastic　wave　propagation　theory,　the　equivalent　elastic　body　for　liquid　sloshing　was　obtained　and　the　liquid　sloshing　can　be　formulated　as　a　dynamic　question　for　elastic　body.　The　3-D　liquid-filled　tank　was　considered　as　a　fluid-structures　interaction　system　that　consists　of　the　ideal　fluid　with　free　surface　and　the　linear,　elastic　solid　with　thin　thickness,　and　the　coupling　equation　based　on　displacement　(structure)-pressure　(fluid　with　free　surface)　format　was　established.　The　static　condensation　method　was　used　for　eigenvalue　problems　of　liquid　sloshing.　Taking　into　account　earthquake　effect　leads　to　additional　load　to　the　coupling　equations,　a　finite　element　model　of　liquid-filled　tank　was　set　up.　The　eigenvalue　problem　and　dynamic　response　of　a　liquid-filled　tank　were　studied　using　the　model.　A　numerical　simulation　of　a　liquid-filled　tank　was　carried　out　to　confirm　the　validity　and　effectiveness　of　the　method.