Traveling wave protection is widely used as the main protection in flexible DC transmission lines. However, as a single-ended protection, traveling wave protection has limited ability to withstand transition resistance. Therefore, given the problem that there is diminished sensitivity in fault detection for main protection of flexible DC transmission lines caused by high-impedance grounding faults, a fast pilot protection method is proposed based on the abrupt change characteristics of current traveling waves. First, by analyzing the propagation process of the line-mode current traveling wave on the line, the mutation characteristics of the line-mode current traveling wave at both ends of the line during internal and external faults are derived. Second, the least square method is used to fit the line-mode current traveling waves to construct main criterion of protection. The it combines with the starting and pole-selection criteria to form the pilot protection method. Finally, a four-terminal flexible DC power grid model is built on the PSCAD/EMTDC platform for simulation verification. Simulation results show that the proposed protection method can quickly and reliably identify faults both within and outside the zone, and withstand 300  transitional resistance and 30 dB Gaussian white noise. It has low requirement on sampling frequency and communication.