Abstract: The weakness of large-scale interconnected power grids is reflected by the transmission sections, which is the key focus of power system operation and monitoring. In order to deal with the problem of accurate and reasonable identification of key transmission sections, a method based on improved vulnerable lines characterization was proposed. Firstly, an index for quantifying the vulnerability of the line was proposed based on the improved transmission betweenness, which combined the line betweenness and power transmission distribution factor. Then, the vulnerable lines were selected and contained in the expected fault branch set. Secondly, the k shortest paths algorithm was used to search the power flow transferred sections for each expected fault branch respectively, and the key transmission sections of the power system that constitutes the topological cut set were selected according to the line outage distribution factor. Finally, the simulation results of the IEEE-39 bus system show that the proposed method does not rely on artificial experience and can accurately and effectively identify the key transmission sections of the system.