High voltage direct current（HVDC） system changes the operating circumstances of the traditional distance protections for transmission lines， and thus affects its operating characteristics. Based on the distance protection theory， it has been found that the effect of HVDC system on distance protection mainly lies on the changes of measured impedance of distance relays for faults with fault resistance caused by equivalent DC system current. The characteristics of equivalent DC system power-frequency current under different types of commutation failure conditions are analyzed. The impacts on distance relays of different AC system conditions are compared. The methods are verified by PSCAD/EMTDC simulations.