Abstract: Aiming at the failure of the protection scheme of traveling wave main protection under the condition of low signal-to-noise ratio and high transition resistance, a backup protection scheme of DC transmission line based on wavelet threshold denoising and improved editing distance algorithm is proposed in this paper. According to the characteristics of large difference in wavelet coefficients between noise signal and fault signal, wavelet denoising method is used to denoise the fault traveling wave signal. The similarity of traveling wave shape on both sides of the line is high in case of external fault, and low in case of internal fault. According to this characteristic, an improved edit distance algorithm is proposed to overcome the influence of data synchronization and outliers, realize the accurate calculation of fault traveling wave similarity on both sides of the line, and establish the fault criterion inside and outside the flexible DC transmission line. The simulation results show that the algorithm can accurately identify the faults inside and outside the area. Compared with the original editing distance algorithm and other two terminal protection schemes, it has stronger anti-noise interference and resistance to transition resistance.