Fluid　-solid　interaction　(FSI)　poses　a　significant　challenge　in　engineering　applications.　Due　to　the　presence　of　offdiagonal　coupling　terms　in　the　matrices,　it　is　difficult　to　use　explicit　time　integration　method　directly.　Consequently,　the　whole　system　often　needs　to　be　solved　using　implicit　method,　albeit　at　the　expense　of　considerably　increased　computational　cost.　In　this　paper,　an　innovative　approach,　namely　an　explicit　-implicit　hybrid　scaled　boundary　finite　element　method　(SBFEM),　is　proposed　to　mitigate　this　issue.　In　this　method,　the　implicit　time　integration　scheme　is　employed　to　tackle　the　coupling　part　between　solid　and　fluid,　while　the　remainder　of　the　system　is　solved　directly　by　explicit　time　integration.　The　solid　and　fluid　domains　are　both　discretized　by　an　automatic　quadtree　mesh　generation,　ensuring　efficient　mesh　size　transitions　and　facilitating　local　mesh　refinement　near　the　interface.　An　overlapping　zone　is　constructed　near　the　interface　for　data　exchange　between　the　explicit　and　implicit　schemes,　which　is　derived　directly　from　the　quadtree　data　structure.　The　elements　in　the　quadtree　mesh　are　treated　as　general　polygons　formulated　by　the　SBFEM.　As　a　result,　the　proposed　method　capitalizes　on　the　advantages　of　both　implicit　and　explicit　techniques,　leading　to　enhanced　computational　efficiency.　Combining　the　quadtree　mesh　with　the　explicit　-implicit　hybrid　SBFEM　allows　for　the　simulation　of　wave　propagation　in　FSI　system　with　complex　geometric　shapes　and　boundary　conditions.　Benchmark　tests　are　conducted　to　verify　the　accuracy　of　the　proposed　method,　followed　by　a　vibration　analysis　of　an　underwater　cavity　to　demonstrate　its　engineering　applicability.