Three-dimensional　nonlinear　dynamic　responses　of　underground　structures　under　blast　loads　induced　by　explosion　of　the　charge　or　weapons　buried　in　soil　were　numerically　simulated　by　use　of　LS-DYNA3D　in　this　paper.　The　dynamic　finite　element　model　consisted　of　soil,　explosion　cavity,　a　composite　structure　buried　in　soil.　The　composite　structure　is　composed　of　reinforced　concrete　liner,　shock　isolation　backfill　cushion　and　surrounding　soil.　The　blast　loads　were　assumed　as　the　empirical　pressure　time-histories　applying　to　the　boundary　of　the　hypothetical　explosion　cavity.　The　viscous-spring　boundaries　were　used　to　simulate　the　semi-infinite　domain　of　the　soil　extending　away　from　the　edges　of　the　mesh.　Comparing　with　the　results　of　the　empirical　formula　of　TM5-855-1,　it　has　been　shown　that　the　computational　model　proposed　in　this　paper　were　reasonable.　The　shock　isolation　effect　of　the　crushable　backpacking　was　also　discussed.