The high controllability and vulnerability of converters impose strict requirements on the speed of relay protection. Therefore, the whole-line quick-action protection using only single-ended information, that is boundary characteristics based protection, has become a research hotspot in recent years. Firstly, the research progress of DC line relay protection using boundary characteristics is summarized, and the existing boundary characteristics are summarized. On this basis, the active boundary which can generate characteristic signal by active response disturbance and the passive boundary which can only generate characteristic signal by passive response disturbance are defined, and the boundary is classified according to the nature and location of the boundary. Finally, the application scope and application value of each kind of boundary characteristics are analyzed. This work is supported by National Natural Science Foundation of China (No. U1766209).