With the development of offshore wind farms to the deep sea, the all-DC wind farm is gradually coming into serious consideration because of its advantages such as no overvoltage problem. For relay protection for collection lines in an all-DC offshore wind farm, this paper first analyzes the fault characteristics with different fault types and location in the DC collection network. The corresponding fault currents are calculated and compared with the simulation values. A single-ended fault line selection method is proposed using the rate of change of transient current (ROCOTC) at the collection bus side, where the difference of the polarity and amplitude of the ROCOTC are used to identify fault type, line and polarity before overcurrent blocking of the converters. By setting an additional line selection process at the bus, even the fault line with a fault point at the end of the line and high grounding resistance can be accurately identified. A DC offshore wind farm simulation is built in PSCAD/EMTDC. A variety of simulation analyses of different fault scenarios is delivered, demonstrating the validity of the proposed fault line selection method.