In flexible DC grids, the rapid rise of fault current requires that the line protection must complete the fault identification within a few milliseconds. Dynamic state estimation based protection (DSEBP) provides a new idea for flexible DC line protection with good performance. However, the operating frequency in the DC grid is 0 Hz. When the DC grid is operating normally, it is difficult to identify the line parameters online to improve the performance of the protection method. This paper proposes a method to identify the frequency-dependent parameters of flexible DC grids based on the characteristic signal injection of half-bridge modular multilevel converter (HB-MMC). The characteristic signal is extracted by the Prony algorithm to calculate the line parameter under different frequencies. Afterwards, the number and position of residues and poles of frequency-dependent parameters are determined using the vector fitting method. Finally, an improved DSEBP is proposed. The simulation shows that the frequency-dependent parameters obtained by the proposed parameter identification method can be used in the improved DSEBP normally, and the identified parameters have better precision.