A method is proposed to evaluate the security of distance backup protection under power flow transfer conditions, providing support for early interruption of cascading tripping caused by subsequent overload. First, based on the relative position between the measured impedance and the protection operation boundary on the impedance plane, the protection’s ability to avoid overload impedance is quantitatively evaluated. In addition to considering branch transmission power, the method further incorporates factors such as branch power factor angle, protection settings, and operation characteristics of the impedance relay to improve evaluation accuracy. Second, the minimum value of the protection’s ability to avoid overload impedance under multiple simultaneous branch outage events is defined as the evaluation index of protection security under power flow transfer conditions. Consequently, the protection security evaluation problem is transformed into an optimization problem, where multiple branch outage events serve as decision variable. A corresponding mathematical model and a solution approach based on binary particle swarm optimization (BPSO) are developed. Experimental results show that the proposed method effectively captures the combined influence of branch power flow and protection characteristics on overall protection security, allowing for more accurate identification of the initial locations where overload tripping may occur.