Abstract: Accurate frequency domain impedance characteristics of the power system can provide valuable information for protection design, resonance point optimization and stability analysis. The intervention-based impedance measurement method can quickly and accurately obtain the frequency domain impedance characteristics. However, when large amount of intervention is injected, it will affect the normal system operation; else if it is too small, it will cause the injected signal to be overwhelmed. In order to study the correlation between the active threshold of the injected impedance measurement and the internal parameters of the system under test, considering noise component, background harmonic and harmonic loss, a method to determine the active threshold of injected disturbance was proposed. Based on the established equivalent model of the system under test, this paper listed the relative error formula to establish the active threshold. Also, the influence of the index on the measurement result was analyzed. The impedance characteristics of the traction network model were measured by both simulation and experiment. Then, the accuracy of the measurement results under the condition of satisfying/not satisfying the qualified index was compared, which validates the correctness and rationality of the proposed metrics and qualification method.