Unipolar grounding faults are the most common faults in flexible DC distribution networks. When a single-pole grounding fault occurs in MMC-type flexible DC distribution networks using small current grounding methods, the short-circuit current is small (mainly distributed capacitance current) and fault identification is difficult. The ground fault detection of flexible DC distribution network is an urgent matter. Phase-to-mode conversion is used to establish a multi-terminal ring-shaped flexible DC distribution network with a single-pole ground fault zero-mode network circuit based on MMC, and the impedance characteristics of the zero-mode network converter and the DC line are analyzed. In the characteristic frequency band of interest, the zero-mode network is simplified and made equivalent, and the relationship between the transient zero-mode current and the zero-mode voltage of each line is obtained. Based on the distribution characteristics of the transient zero-mode current and zero-mode voltage of each line, a directional longitudinal protection method using transient power is designed. The transient power directions at both ends of the fault line are all negative. At least one end of the power direction at both ends of the non-faulty line is positive. Finally, a simulation verification is built on the PSCAD/EMTDC platform. The results show that the protection method is simple and can be implemented reliably. It also has a good ability to withstand transition resistance. This work is supported by the National Natural Science Foundation of China (No. 52077221).