As a backup protection for HVDC lines, the current differential protection needs long time delay to avoid the influence of distributed capacitor current under external faults. To improve the performance of pilot protection, a novel protection principle based on backward traveling wave difference is proposed. Under internal faults, the backward traveling wave at the side away from the fault point and the forward traveling wave at the side near the fault point will not satisfy the transmission function of the DC line. In the case of external faults, the backward traveling wave at the side away from the fault point and the forward traveling wave at the side near the fault point satisfy the transmission function of the DC line. Therefore, according to the transmission function and the forward traveling wave at the side near the fault point, the backward traveling wave at the side away from the fault point can be calculated to compare with the actual/sampled backward traveling wave to discriminate internal and external faults. The proposed principle is verified in PSCAD/EMTDC, and simulation results show that, it can identify the fault reliably and rapidly under different fault scenarios, while having better tolerance to high fault resistance. This work is supported by National Natural Science Foundation of China (No. 51877084), Natural Science Foundation of Hebei Province (No. E2018502063), and the Fundamental Research Funds for the Central Universities (No. 2017MS096).