When High-voltage direct current (HVDC) project operates in monopolar mode or the ground induced current (GIC) produced by solar flare activity, which would cause DC magnetic bias phenomena of transformer or current transformer for protection. While the direct current flow into the power system, an additional impendence would have shown in measured impedance of the impedance relay, which can expand the measured impedance, this will make the impedance relay under-reaching operating. A corresponding analysis system model of distance protection for transmission lines is established in this paper. The expression of measured impedance of the distance relay is derived when considering the DC magnetic bias, meanwhile, in order to intuitionistic weigh the influence of the distance relay caused by DC migration bias, a concept of DC magnetic coefficient is put forward. Through analyzing the operating characteristics of distance protection in different DC coefficient and simulating by using PSCAD/EMTDC simulation software and results are used to verify the validity of the theoretical analysis.