An analysis made of all kinds of single line faults with four jointed parallel lines on the same tower shows that there exist fixed relationships of magnitude and phase between the sequence components E，F，G，H whatever the asymmetrical short circuit fault on a certain line， while there will be different relationships of magnitude and phase between the above four sequence components when the fault occurs on different lines. Based on this finding， a method for faulty line identification during occurrence of a single line fault is proposed. The results of many EMTP simulation tests with both symmetrical and asymmetrical line models show the correctness of the method.