The frequency characteristics of HVDC transmission line and line boundary are analyzed, the fault transient high-frequency current signal will be weakened through line boundary and DC line. The high-frequency components are used as a criterion for single ended transient protection in some literature, but for long lines, DC line for attenuation of high frequency components may be larger than line boundary, which may cause malfunction protection. Further analysis shows that physical boundary has an amplification effect on low frequency components, and DC line has an attenuation effect on low frequency components. Based on multiband energy, a new principle of single ended transient current protection for HVDC transmission line is proposed. The identification criterion of internal and external faults, lightning stroke fault and non lightning stroke fault are based on the energy of 0~1.25 kHz low frequency, energy ratio of high frequency and low frequency, the fault line is selected by the energy ratio of the two lines.