Abstract: For distance protection,there is no need to determine accurate fault location across the whole line,so long as the border between inside faults and outside faults could be accurately distinguished.Because both terminals of HVDC transmission line are connected to DC smoothing reactor so that it has a clear boundary character,a time-domain distance protection method for HVDC transmission line is proposed.The method is based on the distributed parameter line model.It uses the voltage and current at protection location to calculate them at the terminal of the line.And then the fault distance is calculated with the differential equation.The result is used as action criteria of relay protection.Aimed at problems of large fault location error and possibility of protection refusing to act when the neighbor line fault occurs,the two-segment distance protection is proposed.Simulation results show that the protection based on this algorithm is able to distinguish the fault inside from outside and makes the protection act rapidly and stably within the whole line length.