The invention discloses a power flow transfer ratio-based thermal stable key power transmission channel identification method, and belongs to the technical field of power systems and power system automation. The method comprises the steps of performing fault screening on an anticipated fault set by judging a load rate of a monitoring element after a fault and a load rate of a fault element before the fault; calculating a power flow transfer ratio of the monitoring element after the fault for faults in a key fault set, screening main power flow transfer elements according to a power flow transfer ratio threshold value, and further considering a cascade relationship of the elements and total transfer ratio threshold value restriction to determine the main power flow transfer elements; and forming candidate key power transmission channels by the main power flow transfer elements of the key faults and the fault element, estimating thermal stable limit power of the power transmission channels based on the power flow transfer ratio, and determining a key power transmission channel in a candidate set through thermal stable limit power margin. According to the method, the requirement of performing online safety monitoring on the thermal stable key power transmission channel in view of anticipated fault scene identification under current running condition of a power grid can be met.