The DC line fault waveform of a high voltage DC transmission project is analyzed in this paper, and the conclusion that the lightning strike was the action reason of DC line protection is drawn. Based on the retrieval and analysis of lightning data, all the potential causes that can lead the voltage of pole 1 to -535 kV are eliminated one by one. In the meantime, the lightning is also considered to provide the energy which was the reason of voltage variation from +500 kV to -535 kV. At last, optimization strategy of travelling wave protection and voltage derivative protection is proposed based on the characteristics of the lightning back flashover and the lightning shielding failure to prevent the two protection malfunction.