The analysis method of parallel lines inter-line fault for relay protection is presented， which can accurately analyze the influence tendency and boundary of inter-line fault on the main distance element， so as to extract related measurement parameters in order to study the influence. Fault port equivalent impedance is solved with injecting current， and the analysis of different types inter-line fault is dealt with， combining the method of zero sequence current controlling voltage source with the fault boundary conditions. Considering the influence of related factors such as: strength of mutual inductance and support supply of line， line impedance and fault location etc， the accuracy of protection range with the analytical expression is quantitatively analyzed， which provides theoretical basis for the setting and disposition of relay protection. Finally， the analysis result is verified by the example of Guangdong power grid.