Existing line protection methods for multi-terminal direct current (MTDC) systems are constrained by the placement and values of boundary elements. To overcome this limitation, this paper proposes a non-unit DC line protection method based on the normalized backward traveling waves (BTWs) of the 1-mode voltage. Firstly, this article studies the traveling wave characteristics and derives the expressions for the normalized BTWs. Then, the Levenberg-Marquardt algorithm is used for amplitude fitting and normalization calculation. Based on the normalized BTWs, a non-unit protection method is proposed. Finally, the proposed method is evaluated with a simulation model on the PSCAD/EMTDC platform. The results demonstrate that the proposed method can accurately identify faults of different resistances and distances without requiring boundary devices, and is robust against noise disturbances (35 dB).