Since there is no significant difference of the transient component spectral energy between the internal and external faults in distribution line, the transient protection theory is difficult to be applied in the single-ended protection of distribution line. Based on the analysis of the traveling wave of internal and external faults in the distribution line by grid method, the paper proposes a new method connecting the band pass filters at line boundaries in parallel to strengthen the difference of transient voltage spectrum energy between internal and external faults. The results of PSCAD/EMTDC simulation and physical test prove that: when external faults happen, great attenuation occurs after the traveling wave in pass band goes through the band pass filter. But for internal faults, the traveling wave just has little attenuation since it does not go through the band pass filter, thus the difference of transient voltage spectrum energy in the frequency band between internal and external faults is obvious. This method can distinguish internal fault from external faults, and provides an effective theoretical basis and solutions for distribution line single-ended transient protection.