Nowadays, most HVDC transmission lines use the protections developed by ABB or Siemens Company. The two traveling wave protections are simulated and their merits and drawbacks are summarized, which show that these protection schemes can not reliably and sensitively cut out the high impedance ground fault and be vulnerable to external noises interference. At the same time some new ideas about the transient protection of the DC line are introduced; at last several key issues that could advance the reliability and sensitivity of the transient protection for HVDC transmission line are proposed, which will provide a theoretical basis for raising high-quality DC line protection criteria.