Abstract: To study dynamic effect resulted from the initial damage of the structure during progressive collapse, instantaneously applied load method was adopted to perform dynamic response analysis of a single-story steel plane frame with the failed middle column by a simplified model and frame element model respectively. The analytical results show that the dynamic amplifying effect depends only on damping ratio and the failure time of the member when the structure is elastic. The dynamic amplifying factor decreases with the increase of damping ratio or the failure time and the maximum value is 2. When the structure yields and behaves plastically, dynamic amplifying effect is also related to demand-capacity ratio (DCR) and it increases with the increase of DCR until dynamic divergence occurs.