With the continuous development of the power system, the largescale interconnected power grid has greatly increased the allocation of energy resources, but also brought new problems to the synchronous operation of the power system on the mean time. Partial failures of large power grids may spread to the entire network, leading to cascading failures in the power system, which are generally accompanied by largescale transfer of line active power. In view of this, aiming at the power flow transfer situation after the node is removed, this paper gives the concept of node power flow transfer degree. Considering that the node removal may cause the system to be disconnected, the concept of node position importance is also proposed. Meanwhile, based on these two indicators, and borrowing the concept of electrical coupling connectivity, combining with multiattribute decisionmaking method leads to the ranking method of key node comprehensive indicators. Finally, this paper prevents the occurrence of cascading failures or inhibit the largescale propagation of cascading failures by formulating an active defense strategy for the weak node split operation for a specific network structure. For a provincial system in Central China Power Grid, the identification method and active defense scheme mentioned in this paper are applied. The results show the effectiveness of the method and scheme.