Abstract: VSC-MTDC is a new technology that can overcome the shortcomings of current DC transmission technologies. With the engineering application of DC circuit breaker, ultra-high-speed protection, which can quickly and accurately identify line faults, has become a research hotspot. However, the single-ended protection which is widely studied at present belongs to the type of boundary protection. It has some problems, such as low adaptability to primary structure of the system, inability to coordinate the reliability and the sensitivity, etc. The traditional pilot protection is limited by communication capacity and time delay, and it is difficult to be used as a type of main protection. In view of the above problems, based on the analysis of the complex refraction/reflection characteristics of the traveling wave after the DC grid fault, the refraction/reflection ratio was used to establish the protection criterion. The criterion can cover the whole line with high sensitivity. Considering the special topological structure such as loop network, a locking criterion based on the above ratio relationship was constructed to ensure the high reliability of the protection principle. According to the switch-off time of the converter, the suitable line length range to ensure the protection fast-tripping capability was obtained quantitatively. Finally, the proposed protection principle was analyzed and verified by simulation cases.