The most common fault of a flexible DC distribution system is the single-pole grounding fault. After the occurrence of such a fault, the system will have voltage imbalance between the fault and non-fault poles. This poses a threat to the safe and stable operation of the system, and demonstrates the need for the speed and reliability of the protection scheme. This paper first analyzes the fault current characteristics of a flexible DC distribution system after a single-pole grounding fault, and then verifies the correlation of zero-mode current components at both ends of the line when the fault occurs. Finally, a single-pole grounding line selection scheme based on the correlation of zero-mode current at both ends of the line is proposed. This protection scheme makes full use of the inherent transient characteristics of the system after failure and does not need to use boundary elements to construct boundary characteristics. A modular multilevel converter (MMC)-based electromagnetic simulation model of a flexible DC distribution system is built on a real time digital simulator (RTDS) to verify the effectiveness of the scheme. The results show that the proposed route selection scheme can identify fault lines quickly and reliably, and has a strong capability to withstand transition resistance, noise and transmission delay, and is not affected by AC side faults. This work is supported by the Shanxi Province Science and Technology Major Project (No. 20181102028).