Abstract: In this paper, two blockings failures of Qinghai Tibet HVDC project during its pilot operation are introduced, their causes are analyzed, and improved measures of relative DC harmonic protection are discussed. Then the causing mechanism of low order harmonic in DC side and the logic mechanism of DC harmonic protection are introduced. The simulations for all kinds of low order harmonic faults in DC transmission lines are carried out by using PSCAD/EMTDC software, and the harmonics frequency and magnitude of all faults have been determined. In the end, based on the improvement of Qinghai Tibet HVDC project, a kind of new 150Hz harmonic protection is applied as the backup protection of valve short circuit protection. Simulations show that proposed measure can improve the stability of the project, which widens the application area of this new 150Hz protection.