Abstract: Aiming at the problems currently existing in line protection for voltage source converter-based high voltage DC (VSC-HVDC) grids, a double-ended protection principle based on similarity of time-frequency matrix of initial current traveling wave (ICTW) is proposed. First, the fault characteristics of the ICTWs on both sides of the line in the flexible DC grids under internal and external faults are analyzed. It is concluded that within a specific time window, the frequency-domain similarities of the double-ended ICTWs under internal faults are much higher than those under external faults. Then, the double-ended ICTWs are performed S-transform, and the time-frequency matrices are obtained. Then singular value decomposition (SVD) is performed on the matrices. Afterwards, the similarity calculation formula of the double-ended ICTWs is constructed based on the feature matrices, and the internal and external faults can be identified by the value of the similarity. In addition, lightning interference is identified according to the high and low frequency energy ratio of the ICTWs at both ends of the line. Finally, the simulation results under various fault conditions show that the protection scheme does not rely on line boundary elements; it can protect the full length of lines with different lengths; it has stronger robustness against transition resistance and noise. Besides, it satisfies the quick identification requirements of the main protection for VSC-HVDC grids.