The calculation methods of ground fault through transition resistance are studied. Firstly, single-line ground fault through transition resistance is analyzed by symmetrical component method. The six-sequence component theory is used to analyze two parallel lines on the same tower when the parameters are exactly the same. Secondly, a method is presented to analyze two parallel lines of asymmetry parameter on the same tower. According to symmetrical component method, impedance matrix of the lines is decoupled to positive sequence, negative sequence and zero sequence. It comes to a conclusion that positive sequence and negative sequence have already been decoupled, but zero sequence is still coupling. Then, the coupling zero sequence is decoupled to the same directional zero component and the circumfluence zero component based on six sequence component theory in which the zero sequence is divided into the same directional component and the circumfluence component. And system impedance is modified. Finally, the new decoupling method is applied to do fault analysis and figure up the correction coefficient of transition resistance. The PSCAD results prove that the new decoupling method is correct.