Abstract: Three specimens with different stiffness connections (welded flange-bolted web connection, top-seat angle with double web angle connection, and double web angle connection), which comprised three columns and two beams, were statically tested. The failure mode, the mechanical behavior, and the resistance mechanism of the specimens under the condition of progressive collapse were studied. The results indicated that:the specimen with welded flange-bolted web connection was failed because the tension flanges of the beam fractured near the joint; the specimen with top-seat angle with double web angle connection was failed because of the fracture of the tension angle connecting the column and the beam at the bolt holes, and the section with bolt hole of the beam web was failed with severe bearing damage; the specimen with double web angle was failed owing to the damage of double web angles at the bolt hole. Beam-to-column connections had a great influence on anti-collapse performance of assembly. The resistance of the specimen with double web angle connection was mainly provided by the catenary mechanism; for others, the flexural mechanism contributed the mainly resistance in the early stage of loading, and then the dominant force transfer mechanism was converted to the catenary mechanism to resist external loads. Thereinto, the joint rotation and the axial force of the beam of the specimen with both a top-seat angle and a double web angle connection could be fully developed in the later stage, which showed a greater anti-collapse bearing capacity for reserve.