A novel single-phase grounding fault protection scheme for outgoing transmission lines of doubly-fed wind farms based on waveform similarity factor is proposed. The short-circuit current expression of wind turbine port under grid fault is derived, and the characteristics of transient fault current after the faults on wind power outgoing transmission line are analyzed. On this basis, a zero-mode instantaneous model of the wind power outgoing transmission system is established. Then the zero-mode voltage similarity factor is constructed by using the characteristics of the model difference between the internal and external faults to locate the fault. Finally, the simulation verifies that this method can correctly identify the internal and external faults in various single-phase grounding fault conditions, and the result is not affected by fault resistance and current frequency deviation.