Identifying critical transmission linesprone to triggering cascading failures is vitalto ensure the operational security of power system. Existing studies on critical lines overlook N-1 security network constraints and the bidirectional flow transfer effects between target and other lines. This paper proposes a method for identifying critical lines in transmission grid based on integrated flow entropy under N-1 security network constraints, improving accuracy of critical line identification and effectiveness of cascading failure defense.An entropy index with interval-weighted average load rates is established, representing the impact of different N-1 contingencies on the load rates of target lines. A flow distribution entropy index is also built by weighting the interval-average load rate with probabilities of N-2 line outages, measuring the consequences of target line disconnections from load distribution balance. Hence, a comprehensive flow entropy index is constructed to identify critical linesprone to cascading failures more accurately under N-1 security constraints.Based on the IEEE 39-bus system, comparative results show that the proposed method identifies critical lines more accurately. Expanding the critical lines can significantly reduce the risk of load loss due to cascading failures.