According to the features of the HVDC transmission system, this paper presents the possible influence of harmonic and disturbance in HVDC system on the line protection and converter transformer protection on the basis of analyzing on the protection principle, algorithm and digital filter method of the widely used digital protection, on the basis of the EMTDC calculation on the model of k500kV HVDC system , additionally with the dynamic simulation test examples, finally propeses the suggestions on the protections which will be used in theaicinity of the converter stations.