In order to determine the type of HVDC transmission line fault and locate the fault distance accurately.Build a monopole 12 pulse HVDC model based on PSCAD, then simulate faults inside DC line and the faults outside the DC line, that could influence the line protection performance. Take analysis of the voltage signal of faults based on HHT, extract wave head of traveling wave, then determine the failure time. Take analysis of the intrinsic filtering characteristics of smoothing reactor and DC filter, then forming a fault discrimination method based on frequency and energy. If the fault occurred inside the DC line, according to the failure time that has been detected, could calculate the distance of fault with the application of tow sides traveling wave locating method. And proposed HVDC transmission line protection discriminants and action logic. Through simulation calculation results show that the protection method not only could discriminate the fault occurred inside the DC line, but also could fault distance accurately. In case of large transition resistance did not affect the protection performance.