Abstract: The progressive collapse resistance of RC beam-slab substructure under the loss of a penultimate side column is studied through the experimental and numerical analysis. A scaled RC beam-slab substructure is carried out in the laboratory by considering the effect of service loading during collapse. The failure modes, load redistribution mechanism, and load resisting mechanism are investigated and discussed. The experimental results showed that the beam action, compression arch action, catenary action, and tensile membrane action can be mobilized to resist the collapse of RC beam-slab structures under a penultimate column failure scenario. In the initial deformation stage, the flange effects of the slab could enhance the flexural resistance of RC beams. However, the mobilization of tensile membrane action in RC slabs is earlier than that of catenary action in RC beams. In addition, the numerical analysis based on the commercial finite element software LSDYNA showed that the floor slabs increase the yield strength and ultimate bearing capacity of a RC frame up by 65% and 61%, respectively.