The MMC-based ring DC distribution network has high power quality and power supply reliability, as well as good compatibility with distributed generation. However, there are various types of faults in DC distribution networks, and the fault characteristics are complex and closely related to the grounding mode. The sound fault characteristic analysis of DC distribution networks is, therefore, of great significance to the circuit parameter design and protection configuration. Firstly, a ring DC distribution network model based on MMC is constructed, and the main circuit and control strategies of its key equipment are introduced. Secondly, under three grounding modes, the fault paths and characteristics of the pole-to-ground fault on DC lines and phase-to-ground fault on the AC side of the converter are analyzed. The advantage and disadvantage of different grounding modes are discussed. Finally, the DC distribution network model is established in PSCAD/EMTDC to verify the correctness of the theoretical analysis. The simulation results show that the system grounding mode has certain influence on fault characteristics, while relevant analysis supplies reference for fault detection of DC distribution networks. This work is supported by National Key Research and Development Program of China (No. 2016YFB0900203), National Natural Science Foundation of China (No. 51877084), and Natural Science Foundation of Hebei Province (No. E2018502063).