Based on the analysis on the relationship of the positive and the negative sequence impedance， also the positive and negative fault component current under simple and duplicate fault conditions， a cross-country fault identifier is proposed. Under the circumstance of simple fault on double circuit transmission lines， the positive-sequence impedance is equal to the negative-sequence impedance， and the positive fault component current is equal to or greater than the negative circuit. But the relationships above do not exist under the duplicate fault. Based on this criterion， the cross-country fault identifier is designed. EMTP simulations and fieldwork data test results show that the cross-country fault identifier is correct and reliable.