Accurate identification of key transmission lines in power grid cascading accidents is of great significance for the prevention and control of large-scale power supply interruption accidents. Firstly

using the energy function and combined with the modeling method of direct method

the changes of complex electric quantities such as power flow transfer and voltage fluctuation in the accident process are unified into the framework of energy function

and the energy correlation relationship between the lines is accurately quantified. The complex network model of cascading accident analysis is constrcuted

and then

inspired by the PTDF network capability

the above-mentioned complex network model is used as the execution environment

and the concept of line energy flow is proposed to evaluate the critical degree of network lines in the process of cascading accidents. Finally

the improved OPA model is used to simulate the cascading accident in the IEEE-39 node system

and the Pearson correlation coefficient is obtained to verify the validity of the line importance index

which provides reasonable suggestions for security operation and stability control of power grid.