Abstract: Modular Steel Construction (MSC), as a highly prefabricated building, has the advantages of high degree of industrialization, of green energy conservation and of environmental protection. However, the performance of resisting progressive collapse under extreme loads for MSC should be paid special attention due to its limited degree of indeterminacy and low robustness. As the main load transferring path, the behavior of resisting progressive collapse of MSC beam-column substructure needs to be investigated, as it is different with that of traditional steel frames due to its characteristics of multi-beam and multi-column. The research conducts the Pushdown tests and numerical analyses on the MSC beam-column substructures containing the bolted module-to-module joints with corner fittings and investigates the failure mode and mechanism of resisting progressive collapse under a double-column and a single-column failure condition. The results show that the resistance mechanism of a beam-column substructure under these two failure conditions are different. Under the double-column failure condition, the resistance of substructure is mainly provided by beam mechanism and catenary mechanism, and it is significantly affected by the thickness of connecting plate; and the mode of providing resistance changes from beam mechanism to catenary mechanism with the increase of the connecting plate thickness. Under the single-column failure condition, the load is mainly undertaken by the connecting plate, causing it be tensioned and sheared, while the beam and catenary mechanism is less.