Abstract: A substructure is introduced to analyze collapse resistance of the structure, using a rapid effective method to evaluate anti-collapse performance of the structure. The alternate load path method is used and a general steel frame beam-column substructure is chosen as the analytical research object, then a theoretical calculation model of the collapse resistance of beam-column substructure is developed. According to the internal force changes of the beam-column substructure, the whole loading process can be divided into four stages, and at different stages the formulas for vertical bearing capacity-deformation for the asymmetric span beam-column substructure are given. Focus is placed on the mechanical transformation of resistance from flexural mechanism to catenary mechanism, namely the process of beam-column substructure converting from bending to stretch bending and then resisting exterior load mainly by stretching force, and the theoretical calculation formulae are verified by numerical calculations to meet the demands of accuracy for engineering.