The reliable identification of the fault zone is one of the bottlenecks in the DC line protection of a distribution network, which results in the issue of the cooperation between adjacent line protections, as well as preventing DC protections from malfunction after AC faults. The existing faulted-zone identification methods, which relys on the physical boundary formed by smoothing and filtering reactors, are not applicable for DC grids based on the Modular Multilevel Converter (MMC). Therefore, after analyzing fault characteristics of the DC line and considering the cooperation between protections, the DC line protected zones are divided, and the DC line protection scheme, consisting of the main protection and backup protection, is designed. To implement the selectivity, both-terminal current direction information of the protected line is employed to determine the fault location. Finally, the reliability, selectivity and rapidity of the proposed DC line protection are verified by means of the simulation model of a PV integration system and a multi-terminal DC distribution network. This work is supported by National Natural Science Foundation of China (No. 51877084), Natural Science Foundation of Hebei Province (No. E2018502063), and Science and Technology Project of Hebei Electric Power Company (No. kj2017-008).