Based on GPS technology, a new average current method (ACM) of live line measuring the parameters of transmission lines with mutual inductance is proposed. The mathematical model of the new method is explained in detail. Digital simulation results are shown. Finally, an example of live line measuring four 220-kV transmission lines with mutual inductances in South China power grid is given. Comparison of results between ACM and an increment equation method (IEM) is also shown. The field measurement results prove that the new method can meet the needs of live line measurement of transmission lines' inductance parameters.