Abstract: On analyzing two kinds of the existing identification methods of main failure mode, the problem that the reserved candidate failure elements are not all the necessary elements of failure modes has been studied. According to the basic theory of structural mechanics, the effects of failure elements on residual structure internal force has been analyzed, and the relationships have been deduced; According to the relationships, the linear correlation between failure modes which have the same basic failure elements has been proved; and based on the probability theory, it can be concluded that the main failure modes contain only basic failure elements, thus that the necessary elements of main failure modes are exactly the basic elements has been proved. Based on structure geometry analysis theory, failure modes of structural system have been analyzed, and the basic elements range of structural system failure has been discussed. And then using the idea of gradually building structure, the divided method of minimum existence range of the basic elements has been set up. A new method of identifying main failure mode about trusswork has been put forward combined with the branch bounding method. According to examples analysis, it can prove that this method is rational and has high-efficiency, and simultaneously it can ensure no omission of any main failure modes.