Abstract: Initial failure of local elements caused by accidental events may become a starting point of progressive collapse. Diferring from moment frames or bearing wall systems, the truss system widely used in public architectures with large-span is of low redundancy in nature. In order to raise its structural robustness, precise analysis and therefore valid design trategy shall be expected. As a case study, a steel truss roof is deeply investigated in this paper by numerical method, and the internal force redistribution after intial failure, dynamic effect and primay failure modes are studied. By post-intinial failure analysis, it is found there are three kinds of redistributary mechanisms for truss structure: rotation pin mechanism, sliding surface mechanism and long column mechanism. The concept of sensitive element and key element is conceived according to simplified mathematic models, which provides a simple measure to evulate the safety performance for such structural system.