Based on the distributed parameter model of the HVDC transmission line，the compensating voltage at the end of the protection zone is calculated according to the electrical quantities measured at local convert station.A new monopole protection principle can then be constructed with the calculating voltage and the measuring voltage.When a metallic ground fault or low-impedance ground fault occurs in the line，as the compensating voltage at the end of the line has opposite polarity with measuring voltage at the local convert station，the protection criterion can act instantaneously.When a high-impedance ground fault occurs in the line，compensating voltage at the end of the line is lower than the preset threshold which is set according to the transition resistance.And the protection criterion can act in concert with interrelated protection principles.The coordination rule of this protection principle should cooperate with other interrelated protection principles.It is complete，and it can be put into operation during the whole process of fault.Simulation results have testified the feasibility of the principle.