High-resistance faults reduce the sensitivity and reliability of the main protection of flexible DC lines，and seriously affect the safety of the system. In order to solve this problem and further improve the reliability of the protection，this paper proposes a new principle of single-ended protection based on the DC reactor voltage Manhattan distance. Firstly，the reasons for high resistance fault to affect the protection selectivity are analyzed，and the shortcomings of the existing DC reactor voltage protection principle are revealed. Then，by analyzing the transient characteristics of the DC reactor voltage and the blocking effect of the DC reactor，the reasons for the difference in the voltage waveform of the DC reactor during the internal and external faults are explained，and the Manhattan distance is introduced to describe this difference. In this way，an overall protection scheme based on DC reactor voltage Manhattan distance is proposed. This scheme can detect faults only by measuring single-ended DC reactor voltage，which is easy to implement and has good economic efficiency. Finally，a DC transmission system model is built in PSCAD to verify the feasibility and superiority of the protection scheme.