The line boundary of high frequency voltage signal is composed of DC filter and smoothing reactor installed at both ends of HVDC transmission line, which can be used to propose new principle based protections. However, the differences of component parameter and allocation principle affect the high frequency voltage signal attenuation of the line boundary. Thus, analysis of the attenuation characteristic is presented. Based on the present HVDC project component parameter and allocation principle of DC filter and smoothing reactor, this paper studies the high frequency voltage signal attenuation characteristic of HVDC transmission line with different DC filter structures, different arrangements of smoothing reactor, and different component parameters. The results show that HVDC line boundaries with different component parameters and allocation principle cause large attenuation of high frequency voltage signal, and the difference of high frequency voltage signal when internal fault and external fault occur can be used to form the novel HVDC line protections.