Aiming at the problem of identifying the fault line and solving the over-energy caused by the rapid rise of short-circuit current when flexible DC transmission system is in operation a line protection method using the voltage information of the inductor installed at both ends of the transmission line as the fault criterion is proposed. This method determines the fault line and starts the DC circuit breaker by using the change of inductance induced voltage polarity caused by the rise of current after the occurrence of short circuit. This is main protection. When the main protection is rejected the backup protection is started to determine the fault line and disconnect the line by detecting the inductance induced voltage exceeding the setting value. Finally BPI Buffer Protection Identification is used to buffer and absorb the over-current to protect the whole fault line. A three-terminal ring flexible DC transmission system was built on PSCAD/ EMTDC to simulate the occurrence of unipolar short circuit and bipolar short circuit. The simulation results show that the main protection can quickly identify the location of the fault line and cut the line when the fault occurs which improves the sensitivity of fault identification. When the main protection stops the backup protection can start to identify the fault line. The reliability of the whole protection method has been improved.