The rapid rise of fault current in a flexible DC grid has a prominent contradiction with the weak overcurrent capability of power electronics elements. The DC line protection needs to complete the fault identification in a few milliseconds. Accurate parameter identification of transmission lines is very important in improving the performance of relay protection. However, the lack of a stable fundamental frequency in the DC system makes it difficult to obtain the relevant parameters of the transmission line and to realize the protection principle. There is difficulty in obtaining the frequency-dependent parameters of flexible DC lines. Thus a flexible DC line parameter identification method based on characteristic signal injection with a half-bridge modular multilevel converter (HB-MMC) is presented. By changing the control strategy of the converter, a characteristic signal is injected into the transmission line. Then the signal of different frequencies is extracted by fast Fourier transform and the line parameters at the specified frequencies are calculated. Finally, the corresponding amplitude-frequency characteristic curves are fitted according to the frequency-dependent characteristics of different line parameters. The simulation results show that the proposed parameter identification method can accurately fit the DC transmission line frequency-dependent parameters, and the relative error of the whole parameter identification is less than 1.5%.