In this paper, on the basis of verifying the validity of the model, the abilities of collapse resistant, which are beam-column frame, beam-slab substructures without constraint, beam-slab substructures with three sides restrained, were compared through the finite element software to study collapse resistant mechanism of beam-slab substructures under penultimate-external column removal scenario. The research results indicate that when the progressive collapse occurs, catenary and membrane effect of rebar in slab can resist structure deformation, and improve the structure resistance force. Constrain of slab edge can greatly enhance the ability of collapse resistant, but the transverse beams may damage before the double-span beam. Therefore the ability of collapse resistant of transverse beams should be valued in collapse resistant design.