In response to the issue of protection misoperation on flexible HVDC transmission lines due to lightning interference, this paper proposes a lightning interference identification method based on traveling wave cosine similarity for DC transmission lines. This paper first analyzes the propagation process of traveling wave in flexible HVDC transmission lines under different interference. It concludes that the traveling wave pair composed of forward traveling waves on one side and backward traveling waves on the opposite side exhibits low waveform similarity under internal faults, but the same pair shows high similarity under lightning interference conditions. Based on this characteristic, this paper introduces cosine similarity to characterize the similarity of traveling wave waveforms and proposes the lightning interference identification method. The simulation results using PSCAD/EMTDC demonstrate that the proposed method accurately distinguishes between lightning interference and internal faults, and is able to withstand transition resistance and noise interference.