High voltage direct current (HVDC) technology is the ideal solution for long distance bulk power transmission. However, different from AC systems, differential protection of HVDC lines still faces a lot of challenges. We claim that from the perspective of state estimation, the essence of differential protection is to estimate state variables using measured data, of which the validity indicates whether there is a fault. Therefore, the performance of the differential protection is highly related to the model accuracy. Using a detailed distributed parameter line model, this paper developed a novel differential protection method for HVDC lines based on traveling waves with high reliability. The optimal differential point is also discussed to enhance protection rapidity. Experiments demonstrate that the proposed protection could soon detect internal faults with high fault resistance while maintaining reliability when external faults occur.