In　order　to　study　the　behavior　of　composite　frame　under　column　loss,　a　4-span,　single-story　steel-concrete　frame　was　fabricated.　The　specimen　was　tested　under　middle-column　loss.　The　experimental　results　show　that,　the　progressive　collapse　of　composite　frame　under　middle　column　loss　involves　six　stages　consisting　of　elastic　stage,　elastic-plastic　stage,　arch　stage,　plastic　stage,　transient　stage　and　catenary　stage,　in　which　the　structure　sustains　and　redistributes　loads　through　the　mechanism　of　hinge　action　and　catenary　action;　composite　beam　could　bridge　over　the　remaining　elements　and　assure　the　formation　of　catenary　action　while　arch　action　could　enhance　the　resistance　of　structure　under　column　loss;　the　composite　frame　designed　according　to　current　design　standard　possesses　good　progressive　collapse　resistance　under　the　failure　of　key　column.　Based　on　the　test,　a　finite　element　model　was　developed,　whose　results　were　validated　by　comparing　with　the　experimental　results.　This　model　provides　an　efficient　modelling　method　on　progressive　collapse　analysis　of　high-rise　buildings.　Based　on　the　experimental　phenomena　and　results,　a　new　type　of　＇truss-spring＇　arch　model　was　proposed　to　study　the　arch　action　in　composite　frame.　The　vertical　displacement-load　relationship　formula　and　the　vertical　displacement-horizontal　displacement　relationship　formula　were　derived.　The　accuracy　of　these　two　formulae　is　validated　by　comparing　with　the　tests　results.