When a one-phase ground fault occurs in a double end system，distance protection false operation will result owing to the presence of transitional resistance that brings about inaccurate impedance measurement.First，a qualitative analysis is made of the causes of impedance measuring errors due to transition resistance using the phasor diagram when the protection is at the power transmitting side and the power accepting side.To reduce the errors in measuring impedance due to the transtion resistance，a method of replacing the phase of fault current with the phase of negative current at the bus line is proposed based on the full wave Fourier algorithm.And with the traditional impedance measuring method improved，reactance can be calculated with considerable accuracy even during high resistance grounding.EMTDC simulation shows that this method can effectively reduce the influence of transition resistance on impedance measurement and guarantee the accuracy of impedance measurement.