Abstract: At present,the twelve sequence fault component theory is mainly used for four-circuit transmission lines on same tower.However,the calculation is much too complicated because of the high matrix order.Moreover,it is only suitable for parameter symmetric lines.A new method based on the classical symmetrical component method is proposed.First,the strong-electric and weak-magnetic system of mixed-voltage four-circuit lines on the same tower is divided into two parts:the strong-electric equivalent current source system and weak-electric strong-magnetic system.Secondly,the short circuit current provided by the two parts of power supply are calculated respectively.Finally,the two parts of fault current are superposed as the fault current of strong-electric and weak-magnetic systems.Thus complicated calculation of high-order matrix with clear physical concept is avoided while accurate calculation of unearthed fault can be made.The correctness of the method is verified by simulation results via PSCAD/EMTDC.