Abstract: Apart from the nonlinear dynamic analysis, the nonlinear static analysis is also widely adopted to perform the progressive collapse analysis of reinforced concrete (RC) structures, in which a dynamic increase factor or the energy balance approach can be considered to involve the influence of dynamic effects. However, it is possible that a structure is evaluated to be safe in a static procedure but unsafe in a dynamic procedure when the progressive collapse mechanism switches from the compressive arch action to the tensile catenary action due to the removal of a column. In this case, a brittle failure mode may occur as the deformation capacity is much lower in the dynamic procedure. Thus, the dynamic sensitive danger zone (DSDZ) during the transformation process of the progressive collapse mechanism is investigated on the basis of structural stability theory. A two-bar truss system, an RC beam-column substructure, and an RC frame are adopted as numerical examples. The phase plane method is employed to identify the dynamic instability phenomenon in dynamic responses. The relationship between the DSDZ and static and dynamic instabilities are interpreted, as well as the influence of important structural parameters on them. Moreover, the practical approaches or criteria for identifying the dynamic instability and the DSDZ are proposed. According to the results, not only the static instability but also the dynamic instability may occur during the transformation process of the progressive collapse mechanism. Furthermore, the static and the dynamic instabilities are independent. The dynamic instability can be identified by the phase plane method, in which the structure is stable if the phase trajectory converges to the stable focus. Otherwise, the dynamic instability will occur. When only the dynamic instability exists, it is the case of the DSDZ, which is the subregion of the instable zone.