From the perspective of improving the reliability of UHVDC transmission line protection, a single-ended protection method for UHVDC transmission line based on the characteristics of sudden energy waveform is proposed. The superposition principle is used to analyze the energy of the sudden change after the fault. It is found that the sudden energy of the system is zero when the system is in normal operation. After the fault occurs in the DC transmission line, the sudden change energy has obvious changes, and the starting criterion of the protection of the DC transmission line is constructed accordingly. Further analysis of the influence of the DC filter and the smoothing reactor on the sudden energy waveform is done, it is found that their smoothing effect causes the energy dispersed and the waveform flat. The standard deviation coefficient is used to characterize the fluctuation characteristics of the sudden energy waveform, and the distinguishing criterion of internal area fault and external area fault for DC transmission line is constructed accordingly. The fault pole selection criterion is constructed by using the ratio of the standard deviation coefficients of the positive and negative poles, thereby realizing the full-line quick-action protection of the fault pole. The simulation results show that the protection method can reliably distinguish inner and outer faults of the DC line, select fault pole, and protect the full length of UHVDC lines. This work is supported by Joint Project of Natural Science Foundation of Xinjiang Uygur Autonomous Region (No. 2017D01C028).