Abstract: Three steel beam-column assemblies with different span ratio (1:0.6, 1:1.0, 1:1.4), which were comprised of three columns and two beams with the bolt and weld rigid connection, are studied. Through static loading tests, the failure mode, mechanical behavior and resistance mechanism under the condition of progressive collapse are studied. The test results indicate that the specimens show failure characteristics of multiple and intermittent damage. It is initiated by the fracture of beam tension flange near the middle column and then appears discrepancy due to the different span ratio. For the specimen with equal spans, beam tension flanges near side column fracture; for specimens with unequal spans, damage occurs to the tension flange of short beam fractures near corresponding side column and the short beam before long beam. The deformation modes of three specimens are similar, and the process of anti-collapse mechanism undergoes the flexural action phase, flexural and catenary action transition phase and catenary action phase, successively. The specimens present multiple load peak points because of a number of local damage. The load decreases with the span ratio increasing in the flexural action phase. And then the specimen with equal spans could provide higher resistance due to the working together of upper and bottom beams, while the peak points of the bearing capacity of the specimens with unequal spans decrease with the loading displacement increasing because short beam fails before long beam without giving full play to catenary action. This shows that the assembly with equal spans has better transfer mechanism and is relatively beneficial to improving the anti-collapse bearing capability of the structure under large deformation.