Currently， most of the transient protections for high voltage direct current（HVDC） transmission lines utilize the rising rate of traveling wave to constitute the protection criterion. This approach lacks sensitivity information because of the significant attenuation to the high-frequency signal due to the frequency-dependent characteristics of long lines. This paper employs pole-zero method to analyze the frequency characteristics of the transient signal for faulty HVDC transmission line. It is discovered that the transient signals are consist of position main frequency components， which is generated by fault position， as well as boundary frequency components， which is generated by the boundary filters. The different characteristics between internal and external faults are mainly determined by the pass frequency band of the reflection coefficient and the choke frequency band of the refraction coefficient. Therefore， the feature band for distinguish internal and external fault is from the cutoff frequency of the stop band to the highest frequency of the pass band. Due to the zeros of the voltage in the feature band， the fault information in voltage will be counteracted. However， the differences of the current in the feature band are significant for there are no zero points in feature band. The simulation results by power system computer aided design（PSCAD） associated with the typical HVDC transmission system show that the differences of the current between internal and external faults are quite obvious.