The invention relates to a DC power grid line fault protection method. The method comprises steps: travelling wave signals at each measurement point are monitored in real time, if a protected set threshold is reached, a corresponding DC circuit breaker is tripped, all circuit breakers are finally tripped to realize separation of three DC subsystems, and the control mode of each subsystem converter station is switched. According to the travelling waves acquired at the measurement point, whether the fault is subsystem internal fault or subsystem external fault is judged by each subsystem through a fault positioning logic. If the subsystem external fault is judged according to the fault positioning logic, the line of the fault is determined according to the fault positioning logic, or otherwise, the subsystem internal fault is judged. According to an instruction of a dispatching system, the control mode of each converter station and a power control instruction are changed. The technical scheme of the invention can effectively shorten time needed for fault positioning, and power transfer brought by large-range DC power grid locking and influences on the stability of an AC and DC system can be reduced as much as possible. The phenomenon that the disclosed method is not applicable along with complexity of the DC power grid structure does not happen.