The invention discloses an EHV (extra-high voltage) DC grounding electrode line fault monitoring method based on an injection signal, and the method comprises the following steps: S1, injecting a high-frequency sine current signal to the head end of a grounding electrode line of an EHV DC power distribution system through a signal injection device, recording the amplitude and phase of the currentsignal, measuring the amplitude and phase of a co-frequency voltage of a signal injection point at the same time, and calculating the phase difference between the voltage and the current; S2, calculating the wave impedance of the grounding electrode line according to the line parameters at step S1; S3, calculating the voltage standing wave ratio of the grounding electrode line according to the voltage amplitude, current amplitude, line wave impedance and the phase difference between the voltage and current, which are measured at steps S1 and S2; S4, carrying out the recognition of a fault according to the voltage standing wave ratio of the grounding electrode line at step S3.