Abstract: At present, in the dc power distribution network projects at home and abroad, mostly the half-bridge sub-modules are applied and the low-voltage circuit breakers are equipped with to remove faults. However, the measurement errors caused by the fault waveform distortion may degrade the performances of the high-speed main protection or reject the operations. In order to ensure the safe and reliable operation of the flexible DC engineering, it is necessary to study the related backup protection methods. In view of this problem, the time-frequency response characteristics of different diode conduction combinations after the switching device is locked are analyzed, and the fault characteristics of the constant sixth harmonic components are determined. Furthermore, a backup protection based on the sixth harmonic reactance measurement is proposed. The relationship between the protection reactance measurement and the action reactance is used to make the protection operate reliably under various fault conditions. Finally, a dual-terminal flexible DC distribution network model is built on the PSCAD/EMTDC electromagnetic simulation platform. The simulation results show that the proposed protection scheme only uses the single-terminal information to achieve fault removal reliably, and has a strong ability to withstand the transition resistance and noise.