Near-field　underwater　explosion　produces　complex　loading　patterns,　and　complex　boundary　conditions　make　the　damage　patterns　of　structures　under　near-field　underwater　explosion　more　difficult　to　predict.　Thus,　the　investigation　on　the　evolution　of　underwater　explosion　bubbles　and　the　damage　effects　on　the　clamped　square　plates　with　the　coupling　of　multiboundary　(free　surface,　elastoplastic　plates　and　sediment　boundary)　was　conducted　using　the　coupled　Eulerian-Lagrangian　(CEL)　method.　Firstly,　to　verify　the　accuracy　of　the　finite　element　method,　underwater　explosion　tests　were　performed　10　cm　under　the　bottom　of　the　clamped　square　plates　in　different　dimensions　(the　side　lengths　of　the　plates　were　0.46,　0.92　and　1.61　times　the　maximum　bubble　diameter)　using　2.5　g　TNT.　Then,　the　damage　mechanism　of　the　clamped　square　plates　was　analyzed　by　combining　the　test　and　finite　element　results.　Finally,　a　series　of　numerical　simulations　reveal　that　with　increasing　plate　dimension　and　stand-off　distance,　bubbles　show　three　different　evolution　modes:　collapse,　downward　jet,　and　upward　jet.　With　increasing　plate　dimension,　the　effects　of　stand-off　distance　on　the　final　deformation　of　the　plate　center　decreases.　The　sediment　boundary　can　alleviate　the　bubble　shrinkage,　make　the　bubble　firstly　collapse　from　the　middle　to　form　jets　in　the　opposite　direction,　and　reduce　the　displacement　and　strain　of　the　clamped　square　plates.　The　sediment　boundary　has　no　effects　when　the　bubbles　collapse　in　advance.　©　2023　Explosion　and　Shock　Waves.　All　rights　reserved.