Abstract: To address the issues of limited withstand transition resistance and insufficient protection speed of traditional HVDC transmission line protection, a multi-frequency single-ended protection scheme based on line boundary elements is proposed. Firstly, based on the superimposed network, the characteristics of the amplitude ratio of electrical fault components at different frequencies are analyzed. It is obtained that, in the case of internal and external faults, the amplitude ratio of specific high-frequency electrical quantities at the boundary measuring points on one side of the line is different. Based on this, the ratio of multiple specific-frequency electrical fault components is used to construct a criterion to accurately and effectively distinguish internal faults from external faults at the rectifier and inverter sides. Theoretical analysis and simulation verification show that the protection scheme has the advantages of simple principle and small amount of calculation. Internal and external faults can be accurately identified only based on single-terminal electrical quantities. On the basis of ensuring good tolerance to transition resistance, the whole length of the line can be reliably and quickly protected.