Abstract: When monopolar grounding fault occurs in multi-terminal flexible DC system with small current grounding, the voltage of non-grounded pole will rise to twice the rated voltage. However, it is difficult to select the fault line in actual projects. Aiming to solve this problem, we analyzed the characteristics of fault current after a monopolar grounding fault at different positions, and adopted the ratio of the sum of the wavelet noise reduction residual to characterize the difference between the positive and negative fault currents. A multi-terminal flexible DC system monopolar grounding fault line selection scheme was proposed.Communication or separate boundary elements were not needed in the solution scheme. Moreover, a four-terminal ring-shaped flexible DC system model based onmodular multilevel converter (MMC) was constructed in real-time digital simulator (RTDS). Results show that the scheme can reliably identify the faulty line, can tolerate the transition resistance and noise, and is not affected by power flow changes and AC side faults.