High voltage direct current（HVDC） has broad application prospects resulting from energy distribution and network structure in China. The HVDC transmission line is long， and the failure rate is high. Thus the relay performance is of vital importance to the HVDC transmission system security and fault recovery. At present， the operation of the HVDC transmission line protection system and devices are generally provided by ABB or SIEMENS. In engineering application， the HVDC line protection theory is not complete， and the reliability inadequate. As line fault is responded by the DC control system， DC blocking and unnecessary downtime occur more often than not. Based on the state of the art of the protection principle of the classified study， the advantages and disadvantages of principles of protection are reviewed. Finally， several important problems for further study are suggested.