Distributed line model should be adopted for Ultra-High-Voltage (UHV) transmission lines, which cause the replacement linear function with hyperbola function between measuring impedance and fault distance for ground impedance relay. The performance of ground impedance relay is sensitive to earth resistance decided by the characteristics of hyperbola function. Then the bad impact of earth resistance should be evaluated first for the practical application of ground impedance relay in UHV lines. In response to the question, the accurate expression of additional measuring impedance is described, and main factors influencing its characteristic are also analyzed. Theory analysis and case studies prove that the protective zone is reduced largely for relays located both at power sending end and receiving end, which conflicts to the common sense in High-Voltage and Extra-High-Voltage system. The reason is the impact of imaginary part of zero sequence current distribution factor. And the traditional way to overcome the earth resistance with only using the reactance component in measuring impedance is invalid since the non-linearity of resistance and reactance in measuring impedance for UHV lines.