Modular building, referred to prefabricate room units in factory and then transport to site for installation in a Lego-like manner, is an innovative and game-changing technology in building industry. That is exactly what is known as the Prefabricated Prefinished Volumetric Construction (PPVC) in Singapore. Modular steel building (MSB) offers many advantages of especially high industrialization, green energy-saving and being environmental-friendly. However, unlike the conventional onsite building structures, the resisting progressive collapse behavior under extreme events for MSB should be paid special attention due to its discontinuity in the structural frame and diaphragms. This paper focuses on the progressive collapse behavior of the MSB beam-column substructure. First a typical bolt connection with cover plate for interior joint, mentioned in historical literature, has been considered to conduct experimental study. An appropriate experimental method was developed to perform a vertical loading test in order to explore the failure modes and mechanisms of resisting progressive collapse under an interior cluster-column loss scenario. The test results, including failure modes/sequence, load–deformation responses and strain development, are reported in detail, and a thorough analysis of the load transfer mechanism over the entire loading process is conducted.