At present, electrode line impedance supervision (ELIS) based protection is widely used to detect faults on grounding electrode lines, which are indispensable elements of high-voltage direct current (HVDC) systems. The existing theoretical analysis of measured impedance is based on lumped line model and the threshold value is generally set according to engineering experience, which have caused the dead zone problem and even accidents. Therefore, a study on measured impedance of ELIS-based protection and its threshold value selection method is carried out to solve this problem. In this study, the expressions of measured impedance under normal operation and fault conditions are deduced based on rigorous and accurate line model. Based on the expressions, the characteristics of the measured impedance are calculated and analyzed. With the characteristics of the measured impedance, the applicability of the protection with the traditional threshold value is further discussed and the distribution of the dead zone can be located. Then, the method to calculate the threshold value of ELIS-based protection is proposed. With a proper threshold value selected by the proposed method, the dead zone of ELIS-based protection is effectively eliminated, and the protection can identify all types of faults even with large transition resistances. Case studies on PSCAD/EMTDC have been conducted to verify the conclusion.