The backup protection of a transmission line is very important in the hybrid DC transmission system, but existing backup protection is greatly affected by the distributed capacitance and transition resistance. These seriously affect the reliability and speed of the protection. Thus a pilot protection scheme for hybrid three terminal DC lines based on the measured wave impedance phase characteristics is proposed. By analyzing the difference between the measured wave impedance at both ends of the line in the case of internal and external faults of the hybrid DC transmission line, the S-transform is used to extract the initial traveling wave of voltage and current at a single frequency. A criterion area is constructed based on the phase difference of the measured wave impedance for internal and external faults. PSCAD simulation shows that the proposed protection scheme can reliably and quickly identify faults inside and outside the area, has strong ability to withstand transition resistance, and is not affected by fault resistance and distributed capacitance. This effectively improves the reliability and speed of line backup protection.