The　Steel-frame　structure　is　widely　used　in　engineering　construction　due　to　its　main　merits　of　rapid　construction　speed,　excellent　performances　of　ductility　and　light　quality,　etc.　However,　the　capability　of　resistant　incentive　progressive　collapse　under　explosion　and　the　progressive　collapse　mechanics　of　the　steel-frame　structure　is　not　clearly　addressed　up　to　the　present.　Aiming　at　this　issue,　this　study　conducted　a　simulation　analysis　of　progressive　collapse　under　explosion　by　employing　numerical　simulation　method　and　combining　GSA　criteria,　based　on　a　four-story　steel-frame　model　which　was　designed　by　the　specifications　of　Chinese　code.　Therefor,　a　complete　series　of　numerical　calculating　methods　on　progressive　collapse　under　explosion　was　established.　By　adopting　this　series　of　methods,　the　progressive　collapse　mechanism,　structural　members　failure　modes　and　forced　states　were　studied　and　presented　in　this　paper.　The　outcome　showed　that　this　series　provided　with　accuracy,　reliability　and　applicability.　Moreover,　through　the　study,　the　results　indicate:　in　the　process　of　structural　collapse　after　some　partial　components　are　damaged,　failure　modes　of　collapse　forms　and　components　are　greatly　affected　by　catenary　mechanism;　catenary　tension　directly　or　indirectly　results　in　local　warping　and　plastic　instability　of　structural　members.