The converter station in a flexible DC distribution network finds it difficult to withstand the impulse current caused by a bipolar short-circuit fault, and is locked for 1~2 ms. There is little transient effective information for fault treatment, and it has poor location accuracy. Therefore, this paper proposes a fault treatment method of flexible DC distribution network based on DC/DC active injection. When the converter stations are locked, the DC/DC converter at the head of the distribution network will be unlocked in the case of a metal fault, and the DC/DC converter at the head and end will be unlocked successively in the case of a non-metal fault. Then the voltage and current data will be extracted for fault location. The fault location method has considered effective transient information which cannot be provided by the converter station because of fast locking. Based on the electrical characteristic response of the high-voltage side support capacitor of the DC/DC converter to the injection signal, the fault section can be distinguished. Also the method suitable for a network with single MMC converter station does not need additional injection equipment, and the principle is simple and reliable with high positioning accuracy. Finally, the flexible DC distribution network is built in PSCAD/EMTDC to verify the feasibility of the proposed fault location method.