Ultra-high-voltage (UHV) transmission lines have a higher requirement for selecting faulted phases fast and reliably. The coupling relationship between the three phases is analysed in detail. On the basis of the coupling relationship, the principle of faulted phase selection (FPS) is proposed, which utilises the time-domain energy of three-phase transient currents. To remove high-frequency noise and preserve the transient current waveform features invariant, a morphological filter with flat structure element is applied in FPS algorithm. The simulation model of Jindongnan–Nanyang–Jingmen UHV transmission project is established in EMTP. The testing results show that the proposed algorithm is feasible and fast (<2 ms) in case of various fault conditions.