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The line-commutated converter-voltage source converter (LCC-VSC) three-terminal DC transmission system is one of the effective means for large-scale new energy transmission and consumption, and a fast and reliable protection scheme is the key to its reliable operation. However, the LCC-VSC DC system has characteristics such as LCC converter and VSC converter control strategies, fault response, and uneven boundary elements, which makes it difficult to directly apply the existing protection principles. To the above problems, this paper first proposed a single-ended protection principle based on line attenuation characteristics. The protection principle uses the ratio of the high and low-frequency signals of the line mode voltage to express the line attenuation characteristics and uses this ratio to identify internal and external faults. A model of the LCC-VSC three-terminal transmission system is established to verify the feasibility and effectiveness of the proposed protection scheme. The simulation results show that the proposed protection principle can accurately identify internal faults and external faults and has a strong ability to withstand fault resistance and noise interference.