Abstract: In order to accurately identify the key lines of the power grid, the paper firstly proposes the Theil entropy of voltage variation and power flow variation based on the Theil entropy theory, in view of the state of the power grid and with consideration of the types of grid nodes and line load rate. The maximum flow contribution betweenness and average closeness index is proposed, so as to make the evaluation results more consistent with the complex network and the actual grid structure model. the entropy weight method and the analytic hierarchy process are used to determine the weight of indexes, and the comprehensive evaluation index of the key lines are obtained, which not only takes into account the changes of the power grid state and the power grid structure, but also the objective data and subjective experience. The feasibility and practicability of the proposed comprehensive index are verified through the IEEE 39-bus system.