In order to improve the stability of power grid operation and prevent large scale power blackouts

identifying crucial branches in power systems accurately and quickly is of great significance. The theories of complex network and entropy are integrated to evaluate the branch importance from the overall perspectives of structure and running state. Firstly

the index of active power transmission betweenness considering the transmission margin rate is proposed to measure the branch transmission capability and its importance in the topological structure of the power grid. Then

the index of impact based on improved transfer entropy of power flow is proposed to evaluate the power flow impact on the power system caused by branch outages. Finally

an assessment index of synthetical importance of branches is set up to fully measure the importance of branches

including indices of active power transmission betweenness and impact. The test on an IEEE 39-bus system demonstrates the method proposed is valid and rational.