Cascading failures are one of the main causes of blackouts, and fragile lines play an extremely critical role in the evolution of cascading failures. To identify potential fragile lines in cascading failures, based on complex network theory and actual characteristics of power systems, taking line apparent power as line flow, an AC power flow cascading failure model is proposed. Four indicators of electrical in-degree electrical out-degree, and electrical betweenness centrality, as well as weighted power flow transfer entropy are proposed to identify vulnerable lines from the three aspects of network local, global and power flow functional characteristics. Finally, the robustness of the power grid are evaluated through 6 IEEE standard test systems. The results show that the deliberate attack strategy based on the electrical in-degree centrality index will cause the greatest damage to the grid after a cascading failure, and the robustness of the power grid will gradually increase as the network scale increases. This work is supported by the National Natural Science Foundation of China (No. 61773186) and the Graduate Research Innovation Project of Jiangsu Normal University (No. 2020XKT077).