The asymmetry of untransposed transmission lines parameters will affect the correct operation of protection which is based on parameters symmetry. To study the line asymmetry parameters’ impact on distance protection, this paper puts forward the sequence parameter calculation method of untransposed lines. On this basis, it analyzes the error of untransposed lines measured impedance through theoretical derivation and numerical simulation, when the impedance relay is in various fault cases. In response to the measured impedance errors of untransposed lines, improvement suggestions of distance protection setting are put forward.