Abstract: The power system has shown a trend of accelerating the development of power electronics, and the risk of cascading failures of neighboring power grids caused by local power grid voltage instability has increased. Firstly, the temporal and spatial distribution of the node voltage amplitude and phase during voltage instability in a typical source-to-load power transmission system was analyzed. Secondly, the response difference of the branch connected to the nodes at different ends was analyzed, with the simplified branch transient transmission capability (sBTTC) quantitative evaluation index as the characteristic parameter. On this basis, a hierarchical clustering-based method for identifying interconnected branches and cut sets in unstable regions, an active splitting control strategy were also proposed. Finally, for several practical scenarios of AC/DC hybrid power grids with voltage instability problems, the large disturbance simulation results verified the accuracy of the identification of interconnected branches and cut sets, and the effectiveness of active splitting control to reduce the risk of cascading failures.