The smoothing reactor, DC filter and PLC filter form the physical boundary of UHVDC transmission line. The frequency characteristics of UHVDC transmission line and line boundary are analyzed, the attenuation of high frequency fault transient signal caused by UHVDC transmission line boundary and transmission line is studied, and it is concluded that when the distance between the fault position and protector is longer than , the attenuation of transmission line affecting high frequency signal will be greater than that of the boundary. It proves that UHVDC rectifier side and inverter side are symmetrical regarding mid-point of DC transmission line. A double-ended transient based voltage protection principle and operation criterion for UHVDC transmission line which uses protector to distinguish the opposite internal and external fault is proposed. The theory can protect the whole transmission line without direction element. The actual parameter simulation model for Yun-Guang UHVDC transmission system is built up to simulate the double-ended transient based voltage protection method.