The hybrid high-voltage direct current (HVDC) transmission system comprising a line commutated converter (LCC) at the sending end and a voltage source converter (VSC) at the receiving end has broad prospects in many application fields. At present, the protection of HVDC transmission line usually utilizes the transient signals to discriminate faults, and the line boundary characteristics are the key influence factors to the postfault transients. In this paper, the different boundary characteristics at each end of the hybrid HVDC system transmission line are investigated, in both frequency domain and time domain. And suggestions for the transmission line protection are proposed.