Abstract: In the future, the flexible high voltage DC grid is developing in the direction of multi-terminal and large capacity. The use of more economical overhead lines and multi-port hybrid DC circuit breakers (MPBs) is a preferred solution to solve the problem of DC-side fault interruption and isolation. Since the probability of instantaneous fault of overhead lines is much higher than that of cables, it is usually necessary to reclose the fault line after the fault isolation. To avoid the reclosing of MPBs after the occurrence of the line fault, this paper proposes a fault property identification (FPI) method based on MPBs to provide a basis for reclosing decision. The traveling wave (TW) signal is injected into the fault line by controlling the main breaker (MB) of the MPB. If the line fault is instantaneous, the polarity of the first wave of the backward TW is the same as the polarity of the injected TW signal. If the line fault is permanent, the polarity of the first wave of the backward TW is opposite to the polarity of the injected TW signal. Therefore, the FPI criteria can be established based on the above analysis. Simulation results in the four-terminal flexible DC grid simulation model show that the proposed FPI method has high reliability and sensitivity, is easy to be implemented, is robust to fault resistances, and does not have dead zone.