The traveling wave protection and under-voltage protection which are initiated by the voltage derivative can not detect the high impedance fault， and the long operation time of differential protection required to avoid external faults makes it hard to be the back-up protection. Furthermore， the HVDC control system has a great influence on the performance of the DC line protection， especially at the low load level. The differential protection will fail to action due to the current fluctuation caused by the control system. Based on the actual HVDC line parameters and the EMTDC simulation platform， the current DC line protection and the influence of the control system is evaluated， in connection with the fault records and the simulation results. An improvement scheme is proposed.