Abstract: In the flexible DC distribution network based on voltage source converter,the probability of single-pole grounding fault of DC line is much higher than that of short-circuit fault,and the difficulty of identifying single-pole grounding fault increases with the increase of transition resistance.In order to effectively identify the DC line with single-pole grounding fault,the capacitive current characteristics of DC line as well as characteristics of zero（one）-mode network structure after fault were analyzed.The similarity difference of zero（one）-mode pilot current of DC line in case of fault inside and outside the zone was analyzed based on zero（one）-mode network. A single-pole grounding fault protection algorithm for lines in ring shaped flexible DC distribution networks was proposed,which could reflect current similarity.The system simulation model was built in PSCAD/EMTDC to prove the effectiveness and anti-transition resistance of the proposed protection method.