In this paper, three steel beam-column connections (welded flange-bolted web (WFB) connection, welded flange plate (WFP) connection and reduced beam section (RBS) connection) were experimentally examined under impact loadings. The impact tests using drop-hammer were carried out to simulate the dynamic behaviour of structural progressive collapse. The influence of strengthening or reducing methods on the impact resistance of steel frame substructure was discussed. The failure modes and the dynamic responses of the impact load and deformation time histories as well as the dynamic rotation and energy dissipation of the substructures were investigated. Test results showed that the upper flange of all specimens experienced local buckling near the plastic hinge section, while no welding seam cracked. For retrofitted connections, the out-of-plane and distortional deformation of steel beam flange and web at the plastic hinge section were measured. All the three connections with weld access holes recommended by American Federal Emergency Management Agency (FEMA) had good rotation abilities and impact loading resistances, and their ultimate rotations were not less than the FEMA350 design limits. The ductility and energy dissipation of two retrofitted beam-column connections were improved, which were better than those of un-retrofitted connections. The finite element analysis (FEA) models of steel beam-column connections were also established using ABAQUS software. The internal force development of connections during the impact loading was examined and the results showed that the strengthening or reducing methods had a profound influence on the formation of catenary action of beam-column connections under the dynamic progressive collapse scenario.