The DC filter types that are used in actual projects are introduced and its parameter selection methods are also discussed. Moreover， equivalent interference current Ieqfor evaluating DC filter’s performance is introduced and a three-pulse harmonic model for calculating Ieqis presented. Taking Yun-Guang ultra-HVDC project which awaits construction for example， the effects of some factors on Ieqare discussed and analyzed by using the harmonic distributed calculation program HarmDistr developed by Siemens Corporation. Finally， the results of Ieqare compared and analyzed for the different HVDC operation conditions and DC filter’s switching configurations.