Abstract: According to the requirements of DC line protection in flexible DC power grid in terms of speed and reliability, a protection scheme based on single terminal branching coefficient is proposed. The fault current components provided by each converter valve under different fault positions are analyzed, and the branching coefficient is constructed according to the variation of DC line current of single-ended. The results show that the single-termial branching coefficient under the fault in the line protection area is less than that that outside the area. This feature is used to realize the identification of fault area. When the main protection fails due to the high resistance grounding of the line, a longitudinal backup protection scheme using the maximum allowable current of the bridge arm to determine the opening time of the line protection is proposed, so that the problem of timing coordination between the existing main and backup protection schemes is solved, and the uninterrupted operation of the converter under various DC line fault conditions is realized. The protection scheme does not rely on boundary elements and the algorithm is simple. A large number of simulation results show that the scheme has a good ability to withstand the transition resistance and can reliably identify the fault area before the converter locked.