The　equations　of　motion　of　a　mega-sub　control　system　were　established　when　the　sub-structure　was　modeled　as　a　multi-degree　of　freedom　system.　The　transfer　function　of　the　system　was　derived　by　using　the　complex　modal　theory.　The　ultimate　state　function　was　obtained　by　using　the　response　surface　method　and　then　on　the　basis　of　the　first　passage　mechanism　of　random　vibration,　the　calculation　method　of　dynamic　reliability　of　the　system　was　analyzed.　Finally,　the　reliability　sensitivity　of　the　random　parameters　of　the　structure　to　story　drift　angle　was　analyzed　when　the　ground　motion　was　considered　as　a　random　excitation　and　the　structure　was　considered　to　be　in　elastic　state.　The　results　indicate　that　the　mean　stiffness　of　substructure　has　poor　sensitivities　to　story　drift　angle,　while　the　sensitivities　of　the　mean　stiffness　of　main　structure　and　the　damping　ratio　of　isolation　layer　to　story　drift　angle　are　high.　©,　2015,　Chinese　Vibration　Engineering　Society.　All　right　reserved.