Abstract: In the context of equipping impedance monitoring systems for grounding electrode lines in DC transmission systems, the accurate calculation of the fault distance can be achieved by using high-frequency measured impedance characteristics. However,the analysis shows that the grounding electrode line of the DC system may produce the same high-frequency measured impedance under different fault conditions, which means that the measured impedance has not a one-to-one mapping relationship with the fault distance, and it is difficult to obtain the accurate fault distance from a single measured impedance. The paper analyzes the coincidence characteristics of high-frequency measured impedance with the variation of injected signals, proposes a fault location method based on composite-frequency signal injection, and provides a strategy for selecting composite-frequencies. The fault location method based on high-frequency measured impedance amplitude-phase characteristics is improved, and the fault location is calculated accurately. Finally, the feasibility and accuracy of the proposed fault location method are fully verified through simulation cases.